The scale is fine in my opinion. I probably would use more contour lines though (10 meters is more common I think).

By Malicious-Monkey
How long would it take to climb one of the two hills north of the river on foot?
That depends widely of who's climbing... But you could consider something like 40 minutes per 100 meters high for a quite good hiker.

Given the scale, would the middle section of river be considered a ravine or gully or something else?
Hum, hard to tell. What about sketching a side view or slice to check it?