As long as you are confident you are otherwise healthy (heart etc) then there is no reason why you cannot progress at a reasonable rate and start to push yourself.

For bodybuilders who want to burn fat without loosing too much muscle size you are told to do cardio at a pace where you can still hold a conversation. This is obviously to slow if you are trying to get fitter.

Have a search of the web (I'll have a look too) for a chart which will show, by age, what your heart rate should be for a) burning fat or b) improving cardio performance. Sometimes these are printed on the front of the machines too.

In the meantime, for the first few sessions, take it easy and then just add either more distance or up the pace. You will know how hard you are working. Each time just try and better yourself.

I personally like the hill programmes which are a bit like interval training. You work at a steady rate with short sharp periods of increased intensity.

Have fun!