Any training platform that you use will need power data. Strava estimates are ridiculously wrong. Speed is almost useless.
I guess people weren’t able to train before power meters. Obviously other metrics aren’t useless. I doubt pros have a power meter with them on every single training session and I’m sure they actually fail sometimes. I understand training with power data is probably more efficient and better in other ways as well, but to totally discount other metrics, included estimates of power, for a program (Xert) that is “adaptive” seems short-sighted and certainly limits it’s audience. Of course that’s their choice to make. It seems like at the very least other metrics could be used to estimate freshness, for example.
I think you need to accept the fact Xert isn’t for you at this time. Xert’s target market is cyclists who train with power meters on their bikes and trainers.
Xert is constantly monitoring and predicting your fitness level based on real-time power data past, present and future.
The assumption is you are going to be training 5-7 days/week and providing actual power meter data from every workout or data captured during a group ride, event, etc…
Anything less than that and Xert’s Adaptive Training Advisor isn’t going to work correctly.
Estimated power from Point A to B (Strava) is useless for structured training purposes. There is no way to determine how many minutes/seconds you spent in each power zone and when. Remember, it’s called STRUCTURED for a reason. 
If your goal is to experiment with structured training you have a few options.
A. Purchase a power meter so you can monitor and manage ALL rides and train by power zones using Xert, TR, TP, Zwift, etc.
If you plan to use Xert note the “essential conditions” in this article:
http://baronbiosys.com/principles-of-fo cused-progressive-overload/
B. Train by HR zones. This was the standard before power meters came along. It’s not the same thing workout-wise but you’ll get similar results.
https://www.bikeradar.com/gear/article/heart-rate-monitor-training-for-cyclists-28838/
C. Continue to ride by perceived effort putting in 3-4 days of mixed effort miles (hard to do off-road unless you avoid steep climbs and too much technical work) plus a couple of high intensity interval sessions per week (hill repeats if you want). Heck, ride on the road if needed to maintain any timed efforts. Don’t forget a rest day.
http://myworldfromabicycle.blogspot.com/2010/05/do-this-and-you-will-win.html
http://myworldfromabicycle.blogspot.com/2012/04/high-intensity-interval-training.html
Power training is geared towards road riding and indoor trainers. Power monitoring and management off-road is a lot trickier. It can be impractical to follow a structured workout due to long uphill grinds, descents without pedaling, jumping, hopping logs, etc. Watts will jump all over the place instead of timed intervals at specific effort levels that structured training requires.
If off-road rides will be your norm consider options B and C. Start monitoring and managing minutes spent in your HR zones. That alone should provide some structure needed to improve fitness.
You can move up to power training when you can invest in the required equipment and are willing to spend more time on your trainer.
OTOH maybe power isn’t the right choice for you: https://tempocyclist.com/2019/01/17/you-should-not-use-a-powermeter/