Hi Annie,
Glad you're enjoying the program.
To your question, I don't think there's any way to do what you're envisioning. All of the time related computations operate on the timestamps that are associated with each point on a track. If you just draw a track, and add elevation, you won't have any timestamps. So, yes, you pretty much need a GPS to look at average speed, etc.
I could give some thought to giving an option to *add* time data to a GPX file. It probably wouldn't be that hard. Something that might be useful, as well, is a time *predictor* function. Give a route, with elevation, and using mode of transportation (or based on a set of tracks that do have time data) predict the amount of time it would take to cover the terrain.
I'll jot something down in the ideas list to this effect. Thanks for commenting and don't hesitate to ask further questions.