I’ve only tried the river section once in the 10 or so times I’ve done the Borrowdale Bash and came to the same conclusion as you. I guess you have to be pretty fit and have excellent bike skills to make it worth while. I just seemed to end up carrying my bike a lot of the time. However, I tried the Walla Crag start last outing and that is definiteley worth doing along with the Frith Wood diversion.
Another very worthwhile section of trail to add in is going up over the bridleway from little town and up cat bells. Then drop down east to the lakeside again. Great descent, but can be bloody busy.
Bookmarked this thread a few weeks ago. We’re looking to ride Borrowdale this weekend. Will Walla Crag be worth including or will it be too muddy and best avoided?