SPeed and incline is user-dependant. What is fast for you, might be slow for me, what is a high incline for me, might be a low one for you .... make sense? What challenges one, will likely not challenge another.
You have to find YOUR speed and incline. That might also vary depending on the day.
Basically, doing sprints is doing HIIT - High Intensity Interval Training.
There are several threads talking about how to do HIIT. I'll dig some up for you.