The Focus duration and interval target power are determined by the relative amounts of Low, High, & Peak XSS remaining in your buckets. XMB calculates a target that will fill those buckets efficiently and in the appropriate proportions. In other words, if you ride exactly at the target power, your High & Peak buckets should progress proportionally toward completion.
The Peak bucket has a relatively large influence on that target power. Try doing a couple of hard efforts near the beginning of your next ride to pre-fill some Peak XSS and watch what happens to the subsequent XMB target. You’ll likely see it shift downward and the suggested intervals become longer. 
There’s absolutely nothing wrong with following the targets closely if you enjoy doing that! The point is simply that you don’t have to (although, you still sorta need to follow along if you’re trying to fill the buckets, otherwise you’re just doing your own thing anyways 
).
I think this actually highlights an important part of Xert’s philosophy.
It’s very easy to overthink interval structure: Should I do 4×5 or 5×4? Should these be 105%, 110%, or 115%? How long should the recoveries be?
Xert’s approach is that there isn’t one magical combination of intervals that makes you fitter. What matters more broadly is accumulating the intended strain (XSS) across your Low, High, & Peak systems. Magic Buckets gives you a flexible way to achieve that without requiring you to follow a predetermined workout structure (permitting more flexibility in how you train).
Another way to think about it: it’s entirely possible to build excellent fitness without doing traditional workouts at all. Athletes who race or do hard group rides regularly demonstrate this all the time. It’s not the workout structure itself that creates fitness… it’s actually doing the work and accumulating the appropriate training strain.
XMB simply gives us a way to quantify that strain and guide you toward the desired dose of Low, High, and Peak XSS while still giving you considerable freedom in how you get there.
Challenge Level is based on HIE depletion , so both intensity and duration matter.
At the current interval target power, XMB calculates how much HIE you’d need to deplete to reach a Breakthrough, then divides that into 10 Challenge Levels. Roughly speaking:
- CL1 = 10% of the way to a Breakthrough
- CL5 = 50%
- CL10 = Breakthrough
You can therefore “fast start” an interval by going harder than the target initially. That burns through HIE faster and gets you to the higher Challenge Levels sooner. You can then continue the training at those higher CLs, using the fatigue created by the hard start to make the subsequent efforts more effective (since XSS accumulates faster when HIE is depleted/MPA is lowered).
Just keep in mind that going substantially above the target may fill your High & Peak buckets in different proportions than XMB originally calculated, but that’s perfectly okay. XMB constantly interprets what you’ve done and recalculates its guidance on the fly.
You don’t have to fast-start at all. Hold the recommended effort for longer and watch what happens: as HIE continues to deplete, your Challenge Level will continue to rise.
So ultimately, how hard you ride determines how quickly CL rises; how long you keep going determines how far it rises. 
Clear as mud?
the closer you get to your goal, the more specific my intervals become. If you don’t have any event and just trying to get as fit as possible, do micro burst intervals but don’t neglect the longer VO2 intervals. Maybe incorporate them once or twice per month (just to prepare yourself to dig deep mentally)
