The shoes are your biggest problem, the rest of the kit will easily fit into a 10L pack. If you can strap the shoes onto the outside of pack using, e.g. the trekking pole loops, bungee cords or something equally inventive it opens up possibilities.
You'll hear all sorts of different suggestions, I use Salomon, Ultimate Direction, UltrAspire and Lowe Alpine packs depending on how I feel, what I'm carrying and where I'm going. Do you like up-front bottles or a bladder? If you're cycling then maybe up-front bottles aren't suitable?
I'd recommend either finding a store with some in that you can try out, or an online retailer that does free P&P / returns. Narrow down some options, do some ordering and try some out at home. As long as you leave the tags on, don't fill up any water bottles and don't get them dirty returns will be fine. I've done this with Wiggle and Chain Reaction Cycles amongst others - plus they are generally the cheapest place to get them.
Round 1 is my favourite of all the MCN events, there's some really nasty steep descents - especially coming down to the Llanthony CP. it's boggy/muddy up on the second half and last year the final section was a complet mud fest.
I used Inov8 x-talons and was glad I did, if it's dry for the next month, (ha ha ha) you might get away with something less aggressive.
I've done Round 1 three times, and the weather has been epic each time - snow, hail, gales, rain, mud, bogs etc... don't skimp on kit - you have been warned
I signed-up yesterday for the marathon so there's still places left. My 3rd time at Pompey - you can park on-street a couple of hundred yards away from the Pyramids for free, I've never had any problems finding a space