The local tourist office might have suggestions for hiking groups. Taking one of the inexpensive (sometimes tip-only) walking tours might be another opportunity to meet people. Chatting with fellow bus or train travelers can be fruitful, as well.
I've only stayed in a few hostels, and that was 40 (!) years ago, but last year I arranged a two-day trip through Maramures, Romania, through a hostel in Cluj-Napoca. Came across a passing reference to tours organized by that hostel while I was Googling for day-tour opportunities in the area. So I think checking with local hostels about outdoor activities might turn up something you'd enjoy that would also put you in touch with other travelers. Or you might simply meet someone hanging around the hostel reception desk.
Prior to my very long solo trip last year, I had the same concerns you do, but I had enough opportunities to talk to others that I didn't feel lonely. Many times it was English-speaking Europeans whom I was able to catch when they weren't too busy to chat: fellow train-travelers, the night reception clerk at a Serbian hotel, multiple shop keepers in Bulgaria, and a tourist-office staffer in Romania.