Its not as much about building muscle and losing weight but becoming more efficient.30lbs overweight will slow you down but not much IF your already in good cycling shape,100lbs overweight is huge however.I went thru the process of riding lots and losing the weight the old fashioned way,I only lost about 34 lbs,no I wasnt too bad to start with....but I didnt gain any muscle really,just got alot leaner more well defined muscles out of the deal AND lost some muscle elsewhere that wasnt helping me get down the road.Still I think the biggest increases come from better cardio outpu/efficiency and muslces trained to be really efficient at using whats available,not necessarily getting bigger.Takes a LONG time to get in good shape ridng a bike too,many I think neglect that,it can and often takes years to get fast.So,best thing to do is keep riding,riding as much as possible and stay with it.Ive been a messenger for just over 2yrs,yes Im faster than I was when I started,but I still cant hang with the REALLY fast guys,a couple more years of riding 350+ miles/wk and I suspect I may be able to stay with guys like that,it takes time at all levels of cycling to gain speed.Losing weight is just a side effect,ride enough and thats gonna happen no matter what you eat.