You are most likely right, these transmissions have a tendency to burn out after a while.
You can pick up a new transmission from a honda dealer, part number 20001-VG4-H02 It will cost you about $110. These transmissions are not rebuildable, and have no serviceable parts
Remove the spring that holds the belt tight on the transmission then remove the belt and roll the transmission forward. Remove the pulley and cables (the pulley is held on by a roll pin) remove the wheels, and the e-clips that hold the gears on, remove the gears, keys and springs. There will be another e-clip underneath the gear, that needs to be removed as well.On the inside of the deck is one more e-clip that needs to be removed. the transmission can now be slid out of the mower. Reinstall is the same in reverse.
As simple as this job sounds it is not an easy job and unless you feel very comfortable with it then I would suggest that you have a shop do it for you. It should not take a shop more then an hour to complete this.
How do I get that spring off. Does the spring need to come off before I can get the belt off? The belt seems to be tight but could it be possible that just the belt needs replacing? The mower is 10 years old. Your answer to these questions should complete all my questions. Like I said the mower will work some but the drive gets very slow after about 30 minutes.
Your symptoms are exactly the norm for these transmissions. 10 years is a very good length of time for them. There are some fiber clutches inside the transmission that when worn will work when cold and slowly decrease as they warn up. If the belt is tight then it probably does not need to be replaced. Once they stretch they get some slack in them and it is fairly obvious they need replacing.
The easiest way to remove that spring is with a long screwdriver or pry bar. It takes some muscle, but it will pry up and over the top of the bar that holds the transmission belt tight. Yes the spring needs to come off before you remove the belt, unless you cut the belt off.
Do I need to undo the bolt that puts the tension on the spring. It appears there is a nut that I have to hold while turning the bolt head I think is a 10mm
There are no bolts that hold that spring in place. See #4 in the picture, it attached to the rear of the deck by #2 and slips over the big S piece in the front of the transmission.
I apologize you are right, trying to work too fast while I am at work I guess. Yes that bolt needs to come out. Here is "your" transmission.