Man I am so out of shape. I have quite a bit of extra weight on me (5'7", 212lbs - 215lbs) and since it's uncomfortable to run at this point, I opted to get a bike. Much easier. Here's the problem though. Since I've been sitting at a desk everyday for the last 10 years, my legs have gotten weak. I can bike on low resistance for about 5-6 minutes. Sometimes I can do maybe 10 min. It's not that I get winded or tired, it's more that my legs just are weak and give out. I just need to build endurance. What's the best way to build my endurance at this stage?