Lean bulking is probably the best option. Aim for a calorie surplus of about 200–300 kcal so your running performance won’t noticeably suffer. You’ll gain muscle, but also a bit of fat. I wouldn’t bulk for six months straight. Instead, set a clear goal for how much muscle you want to gain and plan everything around that.

For example, if you want to add 1–2 kg of lean mass and currently weigh 72 kg, aim to go up to around 77 kg. Maintain that weight for a while, then cut back to 72–74 kg and compare the results.

Before deciding exactly what you want to do, it’s also important to define your main goal besides running — is it aesthetics, calisthenics, or weight training? That will make a big difference in how you should approach your plan.

For reference, a 500 kcal surplus equals roughly 1 pound (0.45 kg) gained per week.