Pro-hormones will actually probably make your cardio performance worse.
SARMS are going to be your best option as an athlete. Steroids/prohoromones will increase strength, but they will also cause a lot of water retention and negatively impact endurance. Your sports performance will actually probably be worse on the steroids then before you started the cycle.
SARMS on the other hand will directly increase endurance, fat loss and recovery. They aren't as strong as steroids but you get all the good, with none of the side effects. They don't cause water retention, are only minimally suppressive of natural testosterone production and aren't liver toxic.
The SARMS triple stack (S4 + ostarine + cardarine) would be a great first cycle to run:
http://www.evolutionary.org/the-SARMS-triple-stack-protocol
Cardarine will directly increase fat loss and endurance
Ostarine will increase recovery
S4 will increase strength and lean mass
You get the best of everything with the triple stack
After the triple stack, run a mini PCT. 4 weeks of the following:
HCGenerate ES at 5 capsules/day
SARMS are minimally suppressive, there is no risk of aromatization (therefore no estrogen related side effects) and legal to use. You can get everything you need off sarms1.com, n2bm.com and ag-guys.com