Hey everyone,
I have recently moved over to TR from Systm as I really want to follow a long, structured plan, and make some fitness gains. I am only into my second week, however, I was just wondering, is it normal to be losing/just maintaining fitness (according to Intervals.icu) during the base phase? Or is the TSS too low, such that I should be doing additional Zone 2 rides perhaps?
Just to mention, I am on the High Volume XCO MTB Plan, and currently in Base Phase 1.
Cheers.
TR seems to prescribe a lot of endurance rides in the beginning that have an IF in the mid-fifties. If you know what endurance IF you are comfortable with, I’d use Workout Alternates to get yourself into that range. Adaptive Training will eventually catch up your workouts to where you should be (based on your post-workout surveys). But if you know where you should be, I’d jump ahead on the IF and not go flat for weeks while AT catches up.
Hey there! Welcome to the TR community. 
The answer here depends on what you were doing before starting up your current plan. If you were doing more volume/intensity, then it would make sense to see your CTL graph stagnating.
We’d also second what @BrianSpang said – Workout Alternates can be a great tool to use since you already have training history. Your Progression Levels start out at Level 1 if you’re new to TR, which might be too easy – you can use Workout Alternates to find something in the 3.X-4.X range, which is probably more appropriate.
As Brian mentioned as well, Adaptive Training will start to “catch up” as you move through your plan, which will get your workouts and plan more and more dialed in as you go. 
Feel free to let us know if you have any other questions!
I see. Thank you @BrianSpang and @ZackeryWeimer. I’ll give the workout alternates a go.