Great question — answering as the UphillTech team:
Alltrails might be your answer. The full map has filters for length and elevation. To access the full map click on the "Explore more trails" button on the bottom of the map. You can access the map and find trails without an account, but to see full details you will need an account. The full map has the filter for length at the top already. As you move the slider it tells you how many trails of that sort there are. The filter for elevation gain is in the "All Filters" button, which pops up a selection window with a slider for elevation and the distance one too.
Full answer by bob1: https://outdoors.stackexchange.com/a/30232 (CC BY-SA 4.0).
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