You haven't exercised in ages and your whole system is totally out of shape. That defined me about 13 months ago.
In the beginning, your exercise isn't going to do much, if anything at all, to help you lose weight. The reality is that you just aren't going to be capable of burning enough calories in a session to make much impact.
But, there is one good thing about being way out of shape. The more out of shape you are, the faster you improve when you start exercising again. Just do however much you are comfortable doing, as fast as you are comfortable doing it. Do not go over your comfort threshold. All you are trying to do in the beginning is increase your fitness level, and it is going to take a little while (but amazingly not that long).
I think the first time I rode, when I decided to change, I lasted for 7 minutes on a very low intensity, on an indoor exercise bike. I repeated that the next night, though my legs and ***** were killing me. After a few nights I was up to 10 minutes, then 12, then 15 and so on.
When I got to 30 minutes I started increasing the intensity (resistance) while maintaining the same cadence. When I got to the point where I could do 6 miles in 30 minutes I decided to try commuting to my office on a Saturday to see how it went. From that point on I have been a bicycle commuter.
This entire process started in December of 2007. I started commuting in March of 2008.
I don't lose weight quickly. I am not crash dieting or anything like that. There are others here with pretty incredible weight loss stories. For me, I just decided to change the way I live and eat and let it gradually happen. So far I have lost 80 pounds. I would like to be down about 20 more by spring. I don't know if I will reach that or not, but I am definitely going in the right direction.
So, I type all this to basically say, "Don't lose heart." You have to build a fitness foundation. You are doing what you need to do. Just keep at it.