I made a small tool for myself that I thought I would release for other folks who might find it useful too. Frequently I will go for a hike and take many photos without a GPS. While I know where I was, it is difficult to geotag lots of photos at once, because they all correspond to different points along the route I walked.
My tool (http://subject.space/linetime/#/) lets you take a route line from AllTrails, or CalTopo, or other websites (anything in the GPX format) and add time information to it. So, if you know that you got to a certain lake at 11am, and a pass at 3pm, it will interpolate times in between (guessing that you were halfway between the lake and the pass at 1pm.) You can add as many keytimes as you want to make it more accurate.
Then, you can export the route with timestamps and use your photo software (I use Lightroom) to add geotags to images based on the timestamps in the routes.
I hope this is useful to someone!