Yes, definitely lots of lunging. But don't let having an unbroke horse keep you off the trails. I've done "leading hacks" several times before, taking my horse out on the trail in hand. Wear your hiking boots! Some trail class style obstacles on the lead or lungeline can keep your horse mentally fresh, and teaching things like sidepasses from the ground to do the obstacles really helps when teaching them under saddle. Don't underestimate a good rubber currycomb session after your horse's workout, either. Really benefits their skin and coat, and acts like a massage for an athlete. Some daily turnout time to let your horse "be a horse" is a good idea, but I'm assuming your gelding already gets some pasture time. Walking and grazing still builds more muscle than standing in a stall.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown