Here's a page I found when I was looking into body fat. I was interested in how much more weight I would need to lose to get a six pack, never having had one that I remember. (Usually how it goes anyway.) I can see ribs and maybe a beer but definitely no six pack abs. The linked site is commercial in nature but the pics and table are I think accurate. So I personally would need to lose about 8 pounds to get that six pack at BMI 20.2. Not unhealthy 9-10% body fat damn I would feel awful skinny because I weighed that in high school plus I would look like I was in HS with my pants hanging down. A lot of that feeling is a mental thing driven by the upward march of the average American weight. Most six foot guys I meet, most everyone is 200+ lbs. I have one brother my height the others taller, all my nephews are taller, all my male 1st cousins are 6' and maybe 1 or too shorter than me but all over 200.
As far as LD hikers, especially thrus, a BMI of 20 might not provide enough padding when they are not getting enough calories. Also, there is no gender difference in the BMI ranges. Women need a higher base body fat "essential fat". I wonder if that is why the normal BMI range is somewhat wide IMHO. Anyway, as far as the OP and a 20 BMI, while on the lighter side that would keep the weight off the knee and maybe he would like six pack abs if not already present as he does strength training which keeps him current as a firefighter.