I don't recommend going out on 38. I've been down it a few miles, and it is very exposed in that area, sandy, etc. Trail 37 is absolutely horrible as well to climb up. It is really sandy and has a lot telephone-pole water bars everywhere on the climbs.
One thing that I do recommend is trail 40 though, if you have to ride up 37 to get there it can be worth it, but I recommend starting to the north at Williamson Valley Road trailhead, and riding south to the lake, then 349, then around to 37 and back down on 40. This does have exposed sections, but the ride down trail 40 is very fun and roller-coaster like, the ride back the rest of the way from the lake is shallow, but still mostly downhill.
We haven't done this Mt Union ride yet, that is scheduled for Saturday. I have ridden there before though doing shuttles down Yankee Doodle. It is fun, but you are looking at 2200 feet of vertical to get to the top from the bottom. I'd say imposslible on a 45lb bike.
On a 45lb bike about the only thing you can do is ride up thumb butte road and down some fragmented trails and double tracks, although I have ridden my 49lb DH bike from the Williamson Valley trail head a few times, it is doable if you take trail 347 towards the lake. Then either ride back on this or 345. I've even shuttled from the trail 37 start all the way down to Williamson Valley trailhead, I pushed my DH bike up trail 37 for a mostly-downhill ride, although like I said it flattens out over the last half.
I can try to give you some info here, but check pauls book and the trail reviews for better info, the trail reviews themselves are a little limited, but they do have some good suggestions within the posts on how to link some rides.