I'm not a runner. I'm not a bicyclist. I think riding a bicycle on a public road is antisocial, even if it is legal.
What I am is a carhead, and that should give you some idea what I'm about to say.
Years ago, I found a thread on another forum encouraging people to design a racetrack. It wasn't long before someone figured out that your service could be used to map out a racetrack on city streets or open roads.
I don't remember why people didn't just use Google Maps for that. As I recall, there might have been difficulties getting the route to come out right, or saving routes without an account (the latter still persists).
Now you've gone from a pedometer to a "milermeter", and I'm still making auto race routes for trolls 'n' lols. And there are still some difficulties that persist. For most route ideas, the combination of "runners" and "cyclists" options, with a little bit of manual drawing, is sufficient. But, not always. For example, whoever mapped the city of Osaka, Japan was very conscientious about mapping every pedestrian walkway whatever, and in the process made it impossible to make a route on the streets without exclusive use of the "Manually (straight lines)" option. A lot of highways (especially the toll expressways of Japan) are verboten as well. As far as I know, Google Maps still has no way to save large numbers of routes without an account and have them public by default. I also no of no service similar to this, but for mapping drives instead of routes, with that option.
When desiging these tracks, my OCD also does not like to cut against traffic, even if the roads in use would be closed down to race on.
So, my suggestion is to branch out from your current workout-centric setup and implement an "automatically (for cars)" or "adhere to road" option.
Here is an example of one such track I mapped, long before getting this account.
I've seen the occasional incoming link from racing message boards, so I had heard of this happening. I love seeing people use things in ways I never expected. It might not be too hard to make a version of the site that caters to carheads.
Do most people who make these racetracks stick with one-way rules? I'd think if we're designing a sort of fantasy grand prix most people would assume streets would be closed. I like your way better, it's easier since the directions lookups always adhere to one-way rules.
I'd like to bring this back up, as I've found a new thing.
Mt. Desert Island, Maine. Park Ridge Loop and Cadillac Mountain. No matter how hard you try, no matter which option you pick (runners or cyclists), it absolutely will not let you construct a route on the road - you have to use "manually (straight lines)" to get a road route. When you try to place a point, it automatically snaps to a nearby trail, unless there is no trail close enough to snap to, in which case, as I recall, it just refuses to do anything as if you'd tried to route onto a freeway. This despite videos on YouTube showing bicyclists using these roads free of penalty.