Getting Stronger
The strength side of medieval archery is best understood as a very isolated gym session. It's mostly back strength with some arms. Anyone who spends their life ploughing fields, walking with heavy objects to market, and managing a farm with only beasts for power will be ideally suited for shooting heavy medieval longbows.
Training Bands
The most popular small piece of training equipment is the rubber training band. Get the bands with small loops, not the longer Yoga ones, and you'll find it a very good proxy for pulling a longbow.
Gym
Some strong archers maintain their strength at the gym. Typically it's hard to replicate shooting a bow with fixed gym equipment but if you do start shooting heavy bows and are not overly physically active, the general conditioning from lower weight exercises would be beneficial. Typically everything helps but avoid highly isolated movements or single muscle groups as you will look very robotic when shooting a bow.