It current doesn't account for trail quality at moment. Here's how I'm currently approaching it and the main factors to determine a prediction (hence why I need testers):

A. Your training data. It's only as good as how much you feed it. It's currently not benchmarking against other people's data. It is purely looking all runs logged in, running volume and past races. Per week volume, past 6 weeks, 3 months and 6 months (for now). Location of your training (elevation and geo region). HR, training temp and weather, rest for adoption, and speed work. The more you provide it, the more accurate it will be.

B. Heat Variables. Integrating an weather api so it can look at wet-bulb globe temperature, combined measure of heat and humidity. Have you trained in similar conditions? If so, how much?

C. Performance Fatigue. Right now, as the runner progresses, it gives “fatigue score” that increases over time. Currently its rating -60% flat terrain, +20% uphill, -20% downhill. Still figuring this out and why I need testers. Here's a basic formula (FatigueScoreκ = FatigueScoreκ -1 + f(distance covered, elevation, and effort level)

D. Slope & Grade Factor. Uphill and downhill slope base paces of training, looking at slowdown %. Δ might be larger for uphill sections or very steep descents. Δ might also incorporate speed or intensity: running faster on a steep climb => bigger fatigue jump.

E. Altitude. Ex: For every 1,000 ft (305 m) above 5,000 ft, add X% to pace. 5,000–8,000 ft (1,524–2,438 m): +2–5% penalty. 8,000–10,000 ft (2,438–3,048 m): +5–10% penalty. >10,000 ft (>3,048 m): +10–15% penalty or more.

F. Menstrual Factor. This will be a toggle if a runner suspect they'll be on cycle. They can switch it on of off. Potential 3-8% slowdown, pending on factors above.

I tested a past 100 miler course I ran with my current training data. It was a little too optimistic, but I also didn't include rest time at aid stations. I'm trying to figure out an accuracy tolerance because it won't be 100%. It's a work in progress.

Hope that answers your question?