What gearing are you on?
I was once told by a
wise sage that if your legs are tired, drop a gear. If your lungs are tired, go up gear. My problem is my lungs for obvious reasons.
You just gotta ride more to build up your fitness but to make things bit more fun, perhaps you want to get some easier gears?
Also... a lighter bike would make a ton of difference as well. Places like LewMo, a hardtail works well.
Not sure what components are on your bike but those older FS bikes tend to weigh a ton.
Lastly, pick a person and make him your target. It will be a motivating factor so keep on coming out and ride with us.