You are in the lucky generation where this was a thing. It is not a thing anymore. Defined benefit retirement plans are nearly extinct in the US.
Nowadays everybody is on defined contribution plans. The average 401k balance for retiring Americans today is just $180k, which translates to a meagre $7000 per year in retirement income. Worthless.
That's why when you go to Walmart the door greeter is a 80 year old man. He will work until he dies.
Again to the OP: I'd take that bike tour ASAP. 58 planning for 68 is just nuts in my opinion. At 58 you are currently in your last handful of years of reliable good health.
While do doubt some people are, most folks are not victims. Their low 401k balances are of their own design. As mentioned above, and has been said elsewhere, it's not so much about how much you make, it's about how much you spend. If someone decides they want to have an eternal car loan(or lease), large mortgages (or more than one with a second home), take loans on their 401k(AHGCK!!!), carry eternal credit card balances at 20+% interest, "enjoy" a good cigarette and/or booze habit, take pricey vacations, have lots of kids and send them to college or not, ignore employer contributions, fail to invest effort in their own skill set/education level(the days of getting well paid for simply having a pulse are long over)...these are all choices. I returned to the university at age 27 with $50 to my name as any other option looked like a dead end. After 5 years and working 2-3 part time jobs/year I graduated with a degree and a good job offer. A buddy of mine barely got out of high school, is dyslexic, and worked into becoming a machinist for a living. He lives simply, by choice, always has, is retired and is well set up for a good life. Another friend, wonderful person, has been a free spirit her entire life and is proud of it. She's retired..and looking for work..and living with another friend of mine as she can't support herself. We sow and we reap.
With respect to the OP..it sounds like you're locked in with your current time off. Life works out that way sometimes, particularly if you're self employed. I will say, from first-person experience and you can take this to the bank, the degree to which you age between age 48 and 58 is far different than how much you age between 58 and 68. At 66 and in good health I expect I could make it cross-country if I really wanted to, which I really don't. If I were in your shoes, I'd continue to follow your current path, but consider a Plan B as you go along that will satisfy your desires for extended touring. A Plan B may distill out what you're really looking for in a long tour.