Bitterroot 300K route. You could even start in Missoula and do and out an back to the loop.
Highlights include the CdA Trail, the Olympian Trail (assuming you ride to the loop from Missoula), the NorPac Trail and the Hiawatha Trail, which is arguably the most scenic in the country. The two mountain passes you cross are via rail miles, so no killer hills, but not flat. The Hiawatha and the continuation on the former Milwaukee Road right of way is mostly downhill hill for about 25 miles. Maximum elevation (Lookout Pass) is 4,711’.
The CdA is paved. The remaining trail miles are not. I can give you a map to get between Missoula and the loop. Requires a couple of relatively short segments of I-90 riding (legal), but they aren’t bad.
https://friendsofcdatrails.org/route...oot-300k-loop/
From Missoula you could also do an out and back on the 45 mile paved trail to Hamilton. Basically flat with some nice Mountain views.