Ladies, take your own saddle - all the hire bikes have men's saddles. Don't underestimate the difficulty - distances quoted are not huge, but it's a lot more tiring cycling on untarred roads than on tarmac. On the plus side, you will definitely bond with your fellow travellers as you compare notes re sore backsides! May be worth bringing energy bars, if you have favourites - fruit & snacks are supplied regularly during the cycling, but you may want to supplement them. They also provided isotonic mix for adding to water, but again, may not be what you're used to, so if you have a favourite electrolyte powder mix, bring that.
Make sure you check what adaptor you need for Namibian electrical sockets (chunky 2 or 3 pin plugs, not part of most "universal" adaptors - same as South Africa)
It was unusually hot for April when we went, but just in case, bring a long-sleeved lightweight top you are happy cycling in, and possibly longish shorts.