I had the same dilemma with the walking with a 2 1/2 yr old, naturally he walked so slow it was useless. I have since bought him a new bike with a parent control handle on the back, they are a great idea. I went from dawdling to just about powerwalking and jogging with him nagging "faster mummy".
Another good thing I found was exercise videos. Sometimes I will get out the taebo dvd and put it on and my son will come and join in, its hilarious but its a good workout and its just like playing to him.
Double digits by Christmas 08!
GW: 68kg (half my heaviest weight)
This years goals: 120 by 3/10, 110 by 22/11, 99.9 by 25/12, 98 by 1/1/09