Auburn to Old Sacramento via ARBT
Old Town Auburn to Old Sacramento is 48mi each way via the bike trail. Downhill on the way out, 2000ft’ish of climbing on the return.
Move the endpoint to Foresthill Rd and I-80 (park at Raley’s) and it should be 50.
If you don’t want all the climbing loaded on the end, start in Granite Bay and front load it (or I suppose you could park on Auburn Folsom at Shirland Tract/Rattlesnake and start with half the climb).
Good intermediate stops would be anywhere in Old Town Folsom and Bella Bru at Arden (plenty of fast food/gas station options there as well).
If you live in Grass Valley the climb up to Auburn shouldn’t be particularly difficult.