I've got a White outdoors sit on top mower but with a lot of grass to cut it's taking me hours to complete (around 3 acres to cut, some with occasional tress in the way) - takes me around 6 hours total.
Is there any attachment I can get to put on the back that is an extra wide cutting deck?

 

By the time you rigged something up that was viable controllable, and safe, increased the horsepower rating of the existing engine you could buy a 60" ZTR and call it a day.

I formerly cut my 4 1/2 acres with a tractor and finish mower and it took 2 hours, not trimming around any trees, etc due to the limitations. I have a Kubota ZD21 with 60" deck and cut it all, trimming around everything closely in 1 hour 15 minutes. Sold the finish mower

 

It just takes forever on this mower even with sharp blades - not sure why so long.

 

What size deck is it?
There is mowers that you could add but your also doubling the amount of gas and maintaince to run them.
Finish Cut Trail Mowers | Pull Behind & Tow Mowers | Swisher
I have to agree with Chandler, long ago I switched to a Z turn and it's cut my mowing time by 1/2.
Cost a whole lot more to buy a good one but well worth it in the long run.
No riding mower I know of is even rated for a yard that big.
Ferris and Dixie Chopper are great brands offer 0 % interest.

 

Probably not what you want to hear, but I agree completely with Chandler; time for a new mower. I used to spend the better part of every Saturday, when the grass was growing good, mowing. That was with a Bolens lawn tractor. But 21 years ago, before zero turns had taken center stage, I bought a Toro Proline. It's not a zero turn, like it will be replaced with this year or next, but it's a mower, not a lawn tractor, so all of the power goes toward mowing, and I now mow my 3 or so acres of lawn in right around 2 hours. Plus I sold my Lawn Boy push mower later that same year, because I could finally get into places that were to restrictive for a lawn tractor. Chandler mentioned a 60" zero turn, and maybe that fits your budge or maybe it doesn't, but they come in a variety of sizes, and I am certain that you will be money ahead to get an actual lawn mower rather than spending money trying to modify what you have.

 

Probably a 38" deck.
A new zero turn mower could be quite a few thousand $ though? Could almost pay a gardener to do it for the yearly financing? But I suppose used drag behind mowers would be costly too?

 

If you have any unevenness in your yard, tow behind mowers could flip on you. They aren't under your control, only attached to your pull unit. Yeah, a ZTR can cost money, but once you pay that gardener for the year's worth of cutting, what will you do next year?? I was reluctant to put out the money for mine, but wifey suggested it, so I seized the opportunity

 

I agree it's hard to get past that sticker shock.
Many years ago I cheaped out and bought a new JD L 110 to try and mow my 2 acre yard.
I hated that thing, thin stamped metal deck, idler and spring for it breaking, at least 2 new spindles a year, drive belt keeped coming off, deck wheels wearing out.
Second mistake was to buy a new Z 200 John Deere.
Same cheap deck, same issues with parts breaking.
I broke down and bought a new Massey Fergison Z turn.
Huge difference in the way it was made, fabricated thick metal deck, spindles has bigger bearings, cooling fans on the hydro motors, bigger deck wheels with two supports not one cheap stamped metal one like on the John Deere.
I mow at least 13 yards a week with this mower and so far all I've had to do is change the oil and the blades after about 500 hours.
Yes it cost about $5000.00 but I could have bought just one mower instead of having to go through all the cheaper ones first.

 

Dunno, the gardener charges $1100 per year. zero turn is $10,000?
Equally, I cut the 3 acres as I'd rather not spend that $1100

 

Zero turns are not that ridiculous, price-wise; it depends on what you have to contend with. The Ferris that I have my eye on is around $6K, but I cover around three rolling acres, yeah, I know, how much can it roll in three acres, but it does in a few areas, so I want a bit better grade transmissions. A buddy of mine has about an acre and a half, all flat, and the Exmark that he settled on, around $3K, handles it fine. The down side of a lawn tractor versus a lawn mower, for most people, is that they are geared completely differently. The lawn tractor has power going to different places, for tillers, etc., so power to the ground is a lot better, but you don't need that for mowing. If you're mowing, a lawn mower is the way to go. Honestly, I did not take it into consideration at that time, but bet that I cut my fuel cost by at least a third, if not more, when I ditched my lawn tractor and went to a mower. And the cut is at least 10 times better. If you run a snow blower, tiller, and things like that on it, you probably want a lawn tractor. But, if you're mowing, a mower is going to do it more efficiently, both in time and cost, is going to be less costly to maintain, and is going to give you better results.

 

You sure could easily spend $10,000 if you wanted to but 56" Z turn should a whole lot less then that.
I suggested the Ferris brand because of the 4 wheel suspenion.
Any other brand is a rough ride an takes some getting used to.
If the quality mower last 10 years that's about $5500.00 savings over paying some one to do it.

 

Makes sense just wasn't sure how upgrading a 38" to 56" could cut time in half.
Perhaps 38 to 64 and I guess the zero turn helps the turn cuts?

 

Cutting the mowing time is more than just deck width; it also includes ground speed, which is typically going to be faster with a mower than with a tractor, and maneuverability, which eliminates a lot of backing up and going forward and other time consuming activities. And my suggestion is to focus more on efficiency than on deck width. For example, when I bought my Proline, it was available with a 52" deck, and I can't remember for sure now, but I think a 60" deck. For 90% or more of our yard, the wider deck would have been perfect, but I measured some areas around my wife's flower beds, areas around the house, etc., and decided on the 52". The mower is so much more maneuverable than the lawn tractor that by staying a little narrower I also eliminated the need for the push mower for trimming tighter areas, and still knocked my total mowing time down from a full day's worth to a couple of hours. Just as a comparison, my lawn tractor had a 42" deck, so I obviously picked up something in that regard, but I still feel that the majority of the time saving is in being able to get around faster and more efficiently.