I really wouldnt stress about the concept of speed, if you're on a road where it is possible to achieve anything resembling speed, you are either in South Africa or about to be run down by a matatu.
29 inch wheels not a problem most of the chinese bikes have 29 inch rims with skiny-ish tyres, not mountain bike width though.
Take a Garmin GPS with "Tracks4Africa" loaded onto a memory card. I have found that the Power monkey explorer has been invaluable for me.
The route you will have to take in N Kenya will preclude damaging the bike due to speed... Tyres will take a belting and your primary issue will be water.
If you get an opportunity to see a "somali bandit", its a tad late to do anything about it.
About the best you can hope for is to look like a starving native on a bike. I shouldnt have to say it but wear neutral toned clothes that are light green / khaki in colour this will be reasonable camouflage if needed and gets you to more or less blend in.
Take all the stickers off your bike so that it doesnt look new and modern.
Ethiopia is very high Altitude, be aware of that before you start charging off, 80% of Africa's land over 8000ft is in Ethiopia so the odds are you will be exposed to Altitude sickness.
Kenya is great and very welcoming but remember that Africa's oldest tourism hotspot has also got some desperate people looking for anything to get by today on - food, money, water, bicycles, shoes, clothes and sometimes regretfully jig-jigi.
Anything South of Eldoret is likely to be on a tarred road, so no issues there, but you will miss out on allot of the country thats worth going to but not ona push-bike.
If you venture into Uganda be very cautious of a strong banana flavour'd vodka/rum type drink, one bottle-top should be your limit.
Rwanda Burundi... good luck with the Juu juu's they have a habit of creeping up on you - the mind plays funny tricks.
If you venture off any major roads you will face mud. Lots of the glutinous stuff, where even pushing your bike is a massive chore. Some even argue that its easier to lock the brakes and slide the wheels along with all the gunk that will accumulate in your wheels and forks
If you are heading into Tanzania from the south of Burundi beware that there are many many mountains (Volcanoes actually) and I cant see this as being pleasant on a bicycle, the route along the lake shore I dont know if it is open or not now-a-days but a much more enjoyable prospect.
You will need good luck with the Lions if you head over to the East coast south of the Selous.
Alternatively you head south into Zambia but thats a gauntlet I would not want to do unless I were on motorised transport.
Which leaves you with the only sensible option; into Malawi and a very relaxing time of it too (the first day is all down hill then you hit the lake and crash out for a week or so.) getting to this section is a completely different kettle of fish though on a bike.
Getting through East Africa including Kenya and Burundi / Rwanda and Tanzania is challenging and an entire adventure on its own.
If you were looking to do a Long ride from Ethiopia to SA, then i would bypass most of Kenya, because to get from the North is a massive mission alone whereas you can sneak into Uganda South of Lokichogio and its much easier that way through into Rwanda / Burundi. You will have a similar experience but not as much of the hardship.
the East Coast of Lake Turkana is a huge hardship, and once youve hit your lowest low point, it gets harder... for a log time.
Then you get to an oasis paradise (compared to where you will have come from)
Enjoy and dont underestimate the time that you will need.
P.S. Some of your nights will be very very cold, bring a very warm sleeping bag, or a sleeping system that comprises of two bags, a summer and a winter bag.
Oh and remember the wildlife own the nights -
You have a temporary pass during daylight hours, but after dark, try to be bedded down and snoring or it gets to be a very sad story.
I should temper all of the above by saying that i have not done any self supported distance riding in Africa on a push-bike and this is drawn from my experiences in a Landy and on a motorbike.
I used to do a few day long rides on a hard mountain bike 26" with no issues at all, and would imagine that a Front suspension bike "that locks out" would be very welcome.
Most of this was in the Rift valley where there was a support vehicle for the tents and cooking equipment at the end of the day - hell what am I saying, the support team did everything for us, bar the cycling, eating and showering. it was no hardship at all!