I think the length of the original Rider is actually a bit easier to handle because it changes so much over the course of the series, and the main cast keeps changing. Aside from Tachibana, The Shocker Leader, and Taki once he's in the show, and even they aren't technically in every episode, everybody comes and goes and some times they come back (like the Riders themselves!)
You can almost split it in half as two seperate shows, 1~52 and 53~98, and then you can still break it down into a bunch of arcs there. If it seems really intimidating, I'd recommend watching the first 52vand then take a breather, because that's sort of the "intermission" point, and when you come back a couple things are different and it's almost like Kamen Rider 2.0.
Just go into it remembering it is the first one, and they didn't have all the conventions they have now... but damn if it isn't awesome (and my favorite, in case I forgot to mention it again. :laugh: )