My youngest is 3 and a bit and we still use the pushchair. He has been through phases where he hasn't wanted to go in it and has been keen to walk, but now he'll walk for a while and get into the pushchair if he's tired. (Today he had his first afternoon at preschool and I ended up carrying him home as he was exhausted, so tomorrow I'm taking the pushchair!) I have used books and snacks for occupying him in the pushchair, but generally I used to find the worst bit the strapping in, and after that there'd be things to look at once we were moving.
As for whether you'd want a double buggy, I think it depends on how far you walk. If you go most places by car, then you might not need one (although I found it was invaluable being able to strap the toddler in when we were going round the shops), but if, like me, you like to walk to the shops half an hour each way, for example, then I think a double buggy is an essential.