Am I alone in automatically equating “building muscle” with “bulking up” ?? in terms of phrasing, I always heard both interchangeably. Mostly because building muscle usually entails growth.
I’d say strength train 2-3x a week keeping the intensity (weight) high but your reps low (up to 5) I personally find this style of training to be challenging, but rather boring, I HAVE to throw in higher rep days but that’s just me.
Then, sprint at least once a week. You sound conditioned, but doing more than once a week shouldn’t be necessary. Make them hill sprints, at least 10 dashes.