Last years Death Ride the key for me was less about speed than it was heart rate. My Lactate Threshold is about 170 bpm (unscientifically tested by me). I kept my heart rate around 160 bpm all day with minimal spikes close to 170 while standing. I completed the ride without doing only two short rides the four weeks prior to the event due to a non-riding vacation. Obviously your results may vary.
As for speeds I can't say for certain but I'm sure speeds of 5-6 mph are pretty common. I know on shorter training rides with major climbing efforts speeds are between 5-8 mph but again this is on my normal 20 mile weekday training loop. Recently on a 9 mile 5,000 foot gain mountain bike ride I was sadly passed by a runner, pretty quickly actually. And yes I felt like I should just get off and walk.
I think I remember reading in Archie Baker's
ACE Training for Cyclist something to the effect that your cadence should not drop below 50 rpm. If one can not maintain that cadence at a manageable speed then gearing should be changed to allow for a higher cadence. I personally have not followed this rule but thought I would pass it along.
Lastly to increase my power I have taken to climbing my normal hills but in a much harder gear. Something like maybe 34x13 or 14, or 52x15 or 17. My cadence is probably somewhere in the 30 range speeds are probably somewhere in the 4-7 mph range depending on grade. I will mix this routine in with several other training methods.