I think that the way to make it work is for it to be highly customizable. Most running logs are for joggers - there's way to many fields for extraneous bullshit that has little to do with running. I could give a flying f*** about heartrate, weight, which shoes I wore, how much I ran in them, etc.
What would be nice is for you to design a running log that allows the user to determine 1) the form layout and 2) which data fields are in the layout, and 3) which kinds of data reports you want.
For me there would be two default workouts each day. AM and PM. You'd click on AM and a dialog box would pop up with the predetermined data fields "distance" "time" ,When you enter "distance" the cursor then jumps to "time". If you want to enter the time of day, could click on a grayed out data field which would open it up to an active field. Or if you wanted to activate any of your dormant but available fields you could do so with just a click. If not simply click "enter" and the data report calculations would be updated.
If another user generally only worked out once a day, why bother having preset AM and PM workouts already set? If they decided to do doubles one day, just a simple "all workout" button would do it. etc..
If you wanted to add a weight training session simple click on a button and record. Again you could set predetermined data fields or accept the default fields that the developer created.
For interval sessions you would simply select a pre-determined interval data entry dialog box to enter lets say 10 x 400 @60.5. If you wanted to input each of each interval (and rest) simply keep clicking in the interval session dialog box.
1 400 60.7
1 400 61.0
1 400 60.2
1 400 60.9
I think that if you produced a log that is customizable by the user that is completely user friendly and logical, you would have something that you could sell. If not, you just have another completely useless POS running log that you can get at a 1000 places online.