What JJ said: Billygoat '79! As Phil said, vegetation shouldn't be a problem at Mt Tom--the whole place has more deadfall than it did in the old days, but that's the same thing you would experienced any time of year, such as if you were at the recent Troll Cup / Junior Nationals there. The only place where I remember wading through summery vegetation at my run on the course a month or two ago is some ferns on the way from the penultimate to ultimate control.
The FDFs will be out, but that will be true anyplace you do a trail run. I've never tried the blue-hat-with-sticky-goo method, but I've been wearing a buff which takes care of the neck (which is 95% of the FDF problem) and serves as a makeshift fask mask if you need one.
Or what Ian said, but you already knew that. Either Skyline Trail (Blue Hills or Fells) will save you a lot of driving, and maybe the FDFs are a little less bad in the immediate Boston area? Not sure about that. But if you are still catching your toes like you were on Monday, the Skylines are problematic--so many sticking up rocks.
When JJ mentioned Wachusett, I think he was talking about the road, but of course it has trails too. Ari mentioned ski trails but the problem I see with them this time of year is, 1) deep grass and ferns, trouble seeing your footfalls, 2) lack of shade. My map filing system isn't as good as most people's, so I can't quickly pull up a Wachusett O map to find a trail that uses the whole mountain, but just using the recent Campbell training maps, I see a nice 225m climb in 1.5km on the red course map, starting at the ski lodge, passing near red controls 13-14, 10, 6, 4 to the top of a subsidiary summit near red control 3. Rinse and repeat as needed. The training course streamers were supposedly removed on Monday but when I checked just now, the maps
are still online.