I probably would not have chosen to do that with the signs either.
I guess their reasoning could have been.
People vastly underestimate the distance cyclists can achieve. So many people cant believe I cycle to town just to go to the shops, I dont even mention the 200k+ Audaxes that people ride.
So many people are daunted by 2 miles but not by 20 minutes, which is just 6mph.
There has just been a campaign of "five miles to fabulous" To many of us 5 miles is nothing but to many it is too much to consider.
So Sustrans have probably decided it is better to sign in a way that encourages novices, after all us experienced cyclists should be on the road (where we belong, shouldnt we?

)
Also one of the major complaints about Sustrans is that the track surfaces is so variable. If you dont know if you are getting a clear tarmac route or a forest track with 20 stupid cycle barriers on it, then 15 minutes is a lot more informative than 5 miles or 1/2 a mile would have been.
In Wales it is quite common for the height climbed to be more important than the distance cycled. I dont ever see that on the signs. A time would be more useful than a distance in that case.
I do remember when I lived in Australia that it drove me mad that people never gave distances in miles, they always said it in time.
It wasnt what I was used to but it was a more useful system as it tells you directly the fact that you are (in the end) using the distance figures to calculate.