I'm not surprised. When I did that area originally, I tried to stay off the A508 as much as possible as that's a very fast road with very impatient drivers, so to make progress I tended to nip on to the road for as short a distance as possible and get off at the next junction, use the village roads for as long as I could, then only go back to the A508 when I was forced to.
Thankfully the parts nearer Northampton have a cycle path alongside (though that only opened in the last 12 months or so) but the closer you get to Milton Keynes, that's not an option.