How many steps is one run around Prism Tower?
We can use a bit of math:
A Magikarp egg has the lowest number of Egg Cycles (6 cycles), therefore it needs the lowest number of steps to hatch (usually 1530 - 1784 steps).
Get a Magikarp egg, fly to the Prism Tower, start running/riding the bike around the tower and count how many times you do a complete cycle (without using Flame Body, Magma Armor nor the hatching O-Power) before it hatches.
What you are looking for will be something like this:
min_steps_around_Prism_Tower = 1530 / (number_of_rounds)
max_steps_around_Prism_Tower = 1784 / (number_of_rounds)
EDIT:
I did a brief testing with a small sample of 4 Magikarp. Here are the results I got hatching 4 Magikarp eggs:
- 8 and 1/8 rounds
- 7 and 1/2 rounds
- 7 rounds
- 7 rounds
The previous formulae give us (rounded values):
- 188 - 220 steps
- 204 - 238 steps
- 219 - 255 steps
- 219 - 255 steps
The average is a range between [207.5 - 242] steps.
Anyway, if we consider only the lowest distance (the nearest value to 1530 steps), we can assume that is covered in about 7 rounds.
Therefore, this gives us 219 steps per round.
If you make a complete circle at the farthest point with no interruptions (just barely brushing past the boy at the entrance to the plaza by the PokéCenter on the tower side) it is exactly 185 steps. I counted.