I have couple athletes like treadmill but most prefer the road/trails to run. Me, I am a road/trail runner. To me, treadmill environment is too control. Most runners that run treadmill do it at 0-1% incline that doesn’t relate to the real road conditions. Many fine it difficult to adjust to running outside especially during a race. Treadmill running can be good during bad weather in the winter but don’t make it a habit. My home town has great training for runners and triathletes. We have plenty of hill from rolling to steep. Sometimes during a race, I will hear a runner or triathlete complain it is too hilly. When ask where they trained, answer usually track, flat area or treadmill. Now we are in the good weather, go outside and enjoy what mother nature gave us. And say I am going to conquer this hill! Be healthy and safe!