I've looked into this as can confirm that the reason that the Avoid Hills option is only available for cycling on the Make Me a Route feature is due to a limitation with the routing service we use. However, I couldn't see any reason why the Terrain preference was restricted to cycling only, so we've changed this so that it's now also available for walking and running.
I'd need to look into that further Joe and post back. I think the fact that Tererain and Hill preferences are only available for automatically generated cycling routes is a limitation of the routing service we use for the Make Me a Route feature, but I'll look into it.
I notice that under 'make me a route' that for cyling routes there are options about terrain and hills, but for runners and walkers there are none. Also when plotting a route locally, only roads were used, even though there were plenty of footpaths available.