Hi jnutella,
First of all, congrats on making a start.
I'd review how you are positioned on the bike, and maybe get some experienced help from a bike shop or fitter.
If your seat is too low it will work your quads and knees a lot harder than in a higher position, possibly leading to knee problems. There is a limit to how high you go, too, above which you start rolling your hips which then leads to back and pelvic problems. Hence the need in my view for professional and informed assistance if you are older and/or planning on being more than an occasional rider.
As to training the most effectively, I'd either engage a coach or buy a copy of Joe Friel's The Cyclists Training Bible. I've found it to be excellent. It will help you achieve the best result in the time you have, and help keep you away from the dreaded overtraining syndrome - which leads to fatigue, illness, and becoming an ex-cyclist.