There are many mechanisms, triggered by AAS usage, which can work in opposition to your conditioning goals. I only truly understand a few, which I will outline, but there are many more.
1) AAS creates an increase in muscle mass. Quickly. Muscle requires O2. The heart, lungs, blood substrates etc. do not adapt commensurately to this increase in mass.
2) AAS usually results in a higher BP (water retention, hormonal unbalance etc.), and therefore higher RHR.
3) The nervous system excitation caused by most AAS (especially the more androgenic ones), can interfere with the sympathetic and parasympathetic tone of the heart. Another mechanism that can lead to higher RHR, and poorer cardiovascular recovery.
Bear in mind that while some of the top fighters today do use AAS at various points during their training, they use them primarily for recovery. A top level athlete is likely to be genetically gifted when it comes to conditioning, so they can tolerate the loss if they deem it necessary. Most of us do not have that luxury.
Additionally, everyone responds differently. You may have no problems whatsoever. However, I doubt if that will be the case.
If you do encounter conditioning loss on cycle, you will regain it after you cease AAS use. So don't worry too much!
Poly