Most people I know assume you have to be sporty to be a bikepacker. Except if you have particular medical conditions, it couldn't be further from the truth: it's only a matter of pace.
There is no manual telling you to cover 100km every day. It's all up to you, really.
I didn't train to perform my first 500km bike ride. It wasn't easy, but the body quickly adapts to external conditions with proper rest and nutrients. The worst part wasn't even the aching legs or the strained back from carrying a backpack, it was the literal pain in the butt—I didn't wear cycling pants: it was a clothing problem, not a physical fitness one.
That being said, I do am planning more carefully this time.
I re-joined a gym this month to get back on track with my StrongLifts 5x5 program. Barbell squats are the best exercise I can do right now to acquire more strength while maintaining proper biking form, so I'm slowly getting back to my previous squat weight (80kg, weighting 70kg).
I'm getting back to France in about one month, so that's probably when I'll re-settle into a biking routine to activate the specific muscles I'll need. Between 5 and 10km per day at max intensity. I'll be moving around France to see friends and family, so I'll also have to carry my bike with me or simply use it.