Good luck with doing this ride, it's hard work but well worth it. I did it last year with a work colleague, (second time I have done it) but we then went on to Hornsea (wanted to do a coast to coast).
As others have said, whist the terrain is relatively flat (apart from the locks) the surface varies tremendously from good surface to what can only be described as riding across a ploughed field. We did it in 3 days overall, (2 days for the canal). Day one got us a far as Burnley stopping at a very helpful Premier Inn.
First days weather was appalling (heavy rain all day), but managed to get sun burnt on the second!
We had someone hopping along with us in the car, which meant we didn't need to carry lots of provisions. try and do this if you can, and we managed the route with just 4 punctures and one new tyre. (just made Ellis Briggs shop at the side of the Canal)
Would I do it again? Probably. I had a ridged frame bike (not recommended), I have since upgraded to front suspension. A decent saddle is a must as I cannot emphasize the poor surface in some areas. Definitely keep away from the slick tyres, there are more than a few areas where you are riding through mud baths.
We averaged about 12 - 13 miles an hour on the good stretches, down to 7 on the less good and many areas where it was quicker to walk.
I am far from being the fittest person you will ever meet, but getting some miles in before is essential. Feel free to drop me a PM if you want any more info.
Kind regards
Steve