Add to that the fact that you won't have much in the way of brakes, you may be going uphill and travelling at much less than 70, as you said, still a scary experience IMHO
You will have brakes, you just have to exert more pressure on the brake pedal once you exhaust the vacuum in the servo.
As far as scary goes, try L3 of the M6 at 5pm in December with no engine, no electrics whatsoever (and I do mean NO electrics), and flames licking out between the bonnet and the wing 
'kin ell, what happened?
MK II Granada (2.8i Ghia X).
Heard (and felt) something heavy/solid hit the floor of the car when we were on the M1 at Scratchwood. Pulled over to the shoulder to see if anything was amiss (left engine running), but nothing obvious at all.
Journey was fine until J15 or thereabouts of the M6 where the first warning something was amiss was that the engine simply stopped as if the ignition had been switched off.
A few seconds after that, the dashboard went mental in the illumination stakes before total electrical failure struck.
All of the above happened quite literally in the space of a handful of seconds.
I managed to make it to the shoulder without too much trouble (although my brain had certainly been woken up by that time), and as we were moving from L1 to the shoulder thats when I noticed the flames licking out from the side of the bonnet.
Got the car stopped, pulled the bonnet release (left the bonnet down though obviously) and emptied a (now illegal) Halon extinguisher through the gap which achieved the desired result.
Got recovered to where we were going (Barrow) and took a look at what had happened the following day.
Seems the starter motor had fallen out (the earlier noise at Scratchwood must have been one of the starter motor bolts hitting the floor), landed on the subframe, shorted out the positive battery lead, which in turn caused the fire that wrecked the battery, battery leads, and the pipe that fed the oil pressure gauge.
Other damage that was also caused was the starter motor got f*cked when it parted company as it was flicked out by the flywheel, and the gearbox bellhousing was completely fractured all the way around.
I managed to get a shagged secondhand box from a breakers for £20, swapped the bellhousing over, fitted new battery and battery leads.
The local Ford dealer couldn't get me an oil pipe in time for going home, so I stabbed a hole in the oil filler cap and stuffed what was left of my original pipe in that hole.
I have also had the front sprocket fall off my GSX 11 on the M1, although that wasn't as bad as it could have been thanks to the chain I was using (it was too big to allow enough sideways flex for the sprocket to come off the end of the output shaft completely).
About the only motorway incident that has really got the chocolate starfish twitching was when the driveshaft on my GS1000 locked up solid due to a support bearing collapsing, which in turn caused the rear wheel to lock solid.
Again I made it to the shoulder without any damage being caused to machine or us, although that was more than likely down to gods favour rather than any skills I had picked up from my competition days.
After experiencing the shaft lockup, you can now probably guess why something like a simple ignition failure wouldn't phase me in the slightest.