In answer to the original question, 10 lumens is more than enough to navigate by at pedestrian speeds. I've gone through lava tubes with less, and done just fine. If you have to run for your life, maybe not so much. When I had to run, and I wasn't concerned with becoming a beacon for a set of crosshairs, 50 or 60 lumens was pretty much necessary.

In other words, to find the privvy at elk camp at midnight, a small led light such as what DR puts in his pocket survival kit is acceptable. For making a bunker run during a mortar attack at 2:00 am when the generator has quit (again), a G5 Nitrolon was highly desirable.