My resting heart rate is always between 45-50 bpm.
Well your in marathon runner territory then, which sounds about right judging from the amount of running you say you do. The stronger you make your heart obviously the fewer beats you require as each heart beat is more powerful pumping more blood meaning the need for less beats.
I would suggest though if you tried playing football and doing that for 90 minutes you wouldn't feel to good after, as people have said there are all different kinds of fitness. For football a specialist form of circuit training is the norm involving repetitive quick bursts