Really just ride more. I have said this over and over again since joining the forum, ride more, ride to work, ride to your friends house, ride to the newsagent, the pub, the cash machine, to the park, in fact just ride any short routine trips that would normally be a car or public transport journey and also try some longer ones too, they might be easier and more satisfying than you expect. Cycling can be incorporated into a daily routine easier than you might imagine.
Your fitness will follow your increasing activity. It isn't rocket science but it can be fun, rewarding and very satisfying.
im a 63 yr old welder, always done manual work and i used to be up and down ladders but for a while its just stood about at a lathe or welding, gave up cigs 12months ago and i started walking and tried to jog to get more movement in my legs, they were always tired and achey, the bike is part of the overall plan to be fit enough to run a 10k before its too late but i suprisingly have found out that i enjoy cyclingAre you an athlete or have you just started exercising as well as cycling? How old are you?
If you are just starting out, start easy and give your body time to adjust before getting into "training".
Most newbies pedal to hard and too slowly. Gear down and spin up to 60-80 rpm.
Ride at a workrate where you can hold a conversation.
Start with short rides and gradually extend the duration.
After a couple of weeks you will be able to start using the bike for training.
Having stronger legs will not make you a better cyclist. The force we put through the pedals is tiny, it's like climbing a flight of stairs