I hike with a Osprey exos 48. I expect the 38 would be plenty big enough.
https://www.ospreypacks.com/us/en/pr...XOS38_613.html

I use a backpack, and no rack and bags when I hike. With the food and water on the bike as I cycle, the pack is comfortable. An under loaded pack is comfortable on the bike. An over loaded pack is not comfortable on the bike. 2 things; 1. Get a pack with a hip belt. The hip belt takes the weight off your shoulders. 2. Get a pack with a ventilated back, important. My non ventilated pack digs into my shoulders after all day. The non ventilated, no belt, pack makes me sweat so much, the black die dripped out of my shirt and ruined my pants.]

Do not stash to much stuff in the woods while hiking. I ride an old 1995 mt bike to the trail I want to hike. I do not leave $200 worth of bags and stuff on the bike. I do take a photo of the place I stached the bike. I do search for a natural land mark, strange tree stump, pointed rock, something to help me find it again. Last time, I stached it betwen 2 pointed hills. Draw a line betwen the 2 points and return to the bike. I do pick the bike up and carry it to a hiding place, so no one follows my tire tracks to my isolated bicycle. (Rangers in North Cascade National Park found my bicycle and let me know about it 40 miles from the bicycle.)

A front rack with a platform would be better to balance a back pack on. Secure it to the wide handlebars, not the narrow seat.

It took thousands of miles and a few years to wear out an Osprey.