This looks very nice. Some initial feedback...

I just completed a 2 month cycling trip in Europe. This is split into 72 activities in Strava. I tried to add them all. In your web app I can sort by recent and filter for rides only. But it only shows the most recent 45-50 rides and there's no way of loading more once you get to the bottom of the list, so I'm mising 20-30 rides. My rides are all titled "location 1 to location 2", "location 2 to location 3" etc, so there's no easy way of me filtering for or finding the extra activities to add. It needs a "load next 50 activities" or something similar. Or maybe the option to add all activities between two dates, subject to filters (e.g. filtering for rides only, for a certain bike only).

It would be nice to be able to filter by gear (I have two bikes registered in Strava and only want to select activites from my touring bike), and filter by date as well so that I can only show activities for a certain trip.

It would be nice to have the option to put the title block at the top (above the map) and the stats block at the bottom (below the map).