If it goes to 70 degrees, the Mt Vernon Trail will be packed with joggers & walkers...still probably worth doing, but it might require 3+ hours. Beach Drive will also be crowded (it was pretty busy last weekend, despite being in the 40s and damp), but that's a full-on road (closed to cars) rather than a 3.5 foot ribbon of asphalt. Of course, one can always stop in Old Town on the return trip and forget about the crowds over oysters and beer 
If it's 70, I'll be on the trail with a dog leash in one hand and a stroller in the other
