DS is 4 and although we have the carrier still, no way would we let him use a stroller! He can walk 3-4 miles no problem - and has for a good year or so (when so inclined: we got tougher a few months ago). He'll be in the carrier again in a couple of months though as we don't let him loose on Bonfire night - he has to be in the carrier so we don't lose him.

Strollers can be useful to keep a child contained in an area or just for carrying all the children paraphenalia (sp) so it may be worth taking one from that PoV, especially if you have a runner.