For stamina and endurance sport you needs those slow twitch muscle fibres to be well developed. You have either misquoted your coach, or he / she is getting confused. Slow twitch muscle fibres are determined genetically. With training it is possible for some of the fast twitch muscle fibres to take on characteristics of slow twitch fibres.
It's not unusual for your cardio system to be well developed, yet you lack muscle endurance - that sums me up nicely. Lots of steady mileage will help - no need to bust a gut to develop this.
Sorry I think I may have misquoted him. The main jist of it was, that because I was doing alot of strongman work (eg 100kg olympic squat) that my muscles were probably used to those sort of exercising than continuous running.
Yeah that's a very good point actually, I just always imagined my legs would be able to keep up from swimming, cycling, thai boxing etc. But I guess running is a whole new level, I definately notice much more DOMS (delayed onset muscle soreness) the day after running that anything else!