The tunnel is where you go under Connecticut or Calvert (hard to tell by the map). That's where you ought to take the road when the trail crosses around 24th instead of following it along the parkway then left through the zoo (best for a MTB or at least a cross bike, IMO). The sidewalk under the tunnel is too narrow. Not that the trail south of there is great either. I haven't taken the road up this way but I've seen lots of other bikes there. I've taken it home without major incident.

On the southern end, going home (south) is pretty flat (and more downhill on the way back south). With a big enough group especially fairly early, we could go on-road all the way to Virginia Ave, probably.

I think the loop is fun, but that hill is a slow one if you don't know it's coming to get your momentum up. I've never done it the direction you've mapped it, only backwards a couple of times (from the south, Ridge/Ross north to Beach back to bottom of the loop). I've mapped it here: http://www.routeslip.com/map.php?map=14627

(haven't gotten around to doing the whole thing b/c the satellite view doesn't let you see the trail and I haven't mapped the road)