I have a related issue. After watching (as much as I could, before the quality bombed out after 50 minutes) the latest podcost and hearing how we could enter all kinds of things in there, I decided to enter some of my upcoming, qualifying, brevet events. A series of 200, 300, 400 and 600 km is required to qualify for Paris-Brest-Paris in August 2019 (1200+ km in max 90 hours).
The short rides went kind of ok. I used estimations of 10 hours (200 km), 13 hours (300 km), 22 hours (400) and 35 hours (600). I usually do them quicker, but I’ve been known to have these times too.
The TSS actually falls and when I get to 600 km, it bombs out and becomes a negative value. The only way I could get around it was to set the effort to Custom and TSS of 0. Believe me, doing a 600 km ride and aiming for under 30 hours is a bit of an effort, not 0.
So even if it would be difficult to estimate TSS based on distance, it isn’t working based on time either.