One unmentioned cost is the expense to maintain your "at home" life, i.e. mortgage/rent/storage unit, loan payments (if any), phone, car/life/medical insurance, etc. Obviously, these may not apply to you but you would probably have to have money upon your return to get some of them restarted, i.e. rent and rent deposit. I would agree with having access to cash would be very beneficial.

While your 5,000+ mile trip in 60 days is doable, it is not nearly as enjoyable if you can slow down if you choose to. In my younger days, I did the original NT in 50 days (about 84 mpd) and my journal has several mentions how I wished I didn't have to push it (due to weather in the NE). This was after coming up the Pacific Coast so I was in shape so it wasn't a fitness but more of a time in the saddle issue.

No matter what you choose, I wish you a safe and enjoyable trip!