If I get you right, you want to do:
First show the desired speed.
Do that by printing a point or a small line segment at the actual time line (left/rigth for speed differences up/down for time) in PictureBox.
Second you want that line to show also the old values of the speed (i.e. the history).
To do that just "bitBlt" the whole PictureBox nearly onto itself (move it for one Timestep). By that you will see the last time/speed Line/Point moved ontop or under itself. Now you can erase the last printed value and print a new one (each time at the same timeline!). Use "GetTickCount" for the time-schedule.
Now you will have a "waterfall" display of the desired speed.
The next is your "Cursor". Take your value of the tredmill and use the it to place a line object ontop of the picturebox. The Time position should never change.
If you need more help, call back. I used that technic the make a waterfall-display in another application, it works!!
Bye the way, the better running-training is doing it for real! You get fresh air and sometimes even a good look onto other runners (hopefully from the other sex).
My Marathon PB 3:08!!