Yezzy wrote:
I am only 100lbs (155cm)
This is it, and all of it. Unless you gained significant functional (muscle) weight, you should not expect to be great at anything in cycling other than going up very steep and/or long hills (if that). Your genes make you naturally better at running (and apparently swimming), but cycling and dunking basketballs might be more difficult. Sure learning how to handle a bike or getting the best equipment ($$$) might help your average speed, but the core issue, especially in a no-draft event, is power.
The faster you go on a bike, the more of your power is used to overcome wind resistance rather than just moving your weight. Overcoming aerodynamic drag is the main factor at anything over 10mph, so only is less relevant on steep hills. On flats and downhills it's all about air drag, and the power needed to go incrementally faster grows exponentially. At your weight and what you say you can do for 5k, you might be good for 3 or 3.5 w/kg at best for 20min, or about 150w of power on a bike. An equally fit woman (same 5k) weighing 135 would be around 215w. On flat ground that might let you go 18mph, while the other "equally fit" woman might go 22mph. You'll go the same speed only on very steep uphills, and she will fly away on downhills. For you to produce 215w at 100lbs would make you a sub-national level elite female climber (cyclist) and is the ballpark of a 17:00 5k, give or take. There is a reason elite cyclist weights cluster in a range a bit heavier than runners.