PS - we were in Boston Thursday - Tuesday and weren't bored. I like city hopping - the only drawback being you have to pack up and move on every few days. I found walking for a few days a bit tricky, but nothing major.
Aleik, I ran Boston last year and it was actually very warm while we were there. There's plenty to keep you occupied for a few days and it's nice to take it easy and soak up the marathon fever which hits the place.
As others have said, Niagara, NYC and New England are with easy traveling distance - we did them all and had a fab time.
I've done Manchester twice and agree with RunLawrence - nice wide roads, not ridiculously congested makes for plenty of opportunities to take the shortest line. I think I actually run through someone's garden to cut a corner...I didnt realise until I was half way across