Hill work is a good idea, it builds muscle and wind. Take care after your horse's feet, though, on the roads, maybe talk to your farrier and/or vet before getting too gung ho road riding. The concussion, even on a dirt road, can be a lot for a horse's legs and hooves. I would do no more than walk on asphalt or similar roads, anyway. Do lots of lateral flexion. On top of softening and suppling her neck, it builds muscle through the neck and topline, and improves vertical flexion and softness. Encourage her to collect and travel on the bit. Other posters can explain how to achieve this correctly better than I can. Traveling in a collected frame builds the right muscles in the right places. Start slow if your mare doesn't know this yet, she's using muscles that she isn't used to using, and it is a workout for her. Back up lots, in a rounded frame with softness in her face. That builds muscles that horses living in a more natural state seldom use. Other than that, ride as much as possible. It doesn't have to be hard, fast work. The guidelines I was taught are walking = muscle, trotting = muscle and wind, cantering = wind. Long, easy rides and wet saddle pads are your new best friends. Remember to mix it up and keep those long rides enjoyable for both of you. This is good bonding time. Hope that helps!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown