The qualifying contact must be 50km at least (not cumulative) and the award will be issued in steps of 50km (50km, 100km, 150km etc) again not cumulative. So a contact of 80km will qualify for the 50km award and one of 102km for the 100km award etc. but the two togther will not qualify for the 150km award.
I had hoped this was clear enough but this means you may only claim one award step per day. If you make two contacts one of 55km and one of 105km on the same day you may chose whether to claim the 50km award or the 100km award, not both (nor the 150km one). But the rule means that you may go back to the same hill every day of the year and, in theory, claim 365 variants of the award! This is, of course, a total variation on the normal SOTA activation awards where the SUMMIT may only be claimed once per calendar year for points purposes.
In your ski run example, provided the summit was a SOTA summit then you may activate the summit as often as you wish in one calendar day but may only claim one award - the choice of which being left to the activator. The total distance being irrelevant (see above about distances being non cumulative).
For the sake of clarity this also applies to bands, so if your activation was on 23cm and 13cm you may only claim one band-distance award per calendar day not (for example) a 50km-23cm and a 100km-13cm award.
K6EL is in the process of helping the MT formulate the Rules for this award into language that is, hopefully, unambiguous and I hope to publish this version shortly.
I have a sneaking suspicion that my answer above is as clear as mud