This year's event is quite unlike recent years, as there are extended undulating sections. Surprisingly, though, analysis shows that the secret is in the climbs! That extra bit of effort on the climbs will allow you far more time for the long undulating parts.
As usual, the downhill sections are not quite as important. This is because you spend little time descending anyway, so any savings do not amount to much. This is probably as well as the descents are mainly narrow and twisty this year.
I have done some sums based on finishing the course in 10 hours. (harder than it sounds this year!)

Supposing you decide to climb at a moderate rate

Terrain Length Typical Speed Time
Climbs 17 miles - 3h 48m
Descents 25 miles 25 mph 1 hours
Rests     1 hour
Undulating 82 miles 19.4 mph 4h 14m
Total Time     10h 2m

This is based on 150 Watts climbing power for a rider + bike weighing 200lb (12 stone rider + 32lb of bike,clothes,water etc). Covering 82 miles of undulating terrain at 19.4mph would take some doing!

If you ware wondering what 150W represents, or what the climbing speed was - all will be revealed when you read on!

Increasing your climbing power to 200W gives you far more time for the undulating section, so your speed on these sections becomes more feasible.

Terrain Length Typical Speed Time
Climbs 17 miles - 2hr 48
Descents 25 miles 25 mph 1 hour
Rests     1 hour
Undulating 82 miles 15.76 mph 5hr 12m
Total Time     10hrs

The extra effort put into climbing means that you only need to do the undulating section at 15.76 mph average. This is far more feasible. In fact, with the help of the huge pelotons, you may be able to acheive a better average speed on the undulating sections than this. As always there is a happy medium. A good climbing power might be 175 Watts. We then get:

Terrain Length Typical Speed Time
Climbs 17 miles - 3hr 14m
Descents 25 miles 25 mph 1 hour
Rests     1 hour
Undulating 82 miles 17.2 mph 4hr 46m
Total Time     10hrs

This probably represents a good compromise target for the many riders who's target is to complete the Etape successfully. Climbing at these speeds for long periods requires a fair amount of training for most people!

One way to measure your wattage is on an exercise bike or turbo trainer with a well calibrated power meter.
Alternatively, find a convenient long hill with a steady gradient, cycle up it! Then find the gradient from an OS map (rise in meters/distance in meters). Alternatively some hills have gradients marked, or there are other simple ways of measuring gradients
Then use the simple formula:

Power (Watts) = 2 x Weight(lb) x Speed(mph) x Gradient (as a fraction)

For example, suppose you weigh 200lb with your bike, and want to climb a 7.5% gradient at 5mph.

we get:

2 x 200(lb) x 5(mph) x 0.075(gradient) = 150 (Watts)

This formula will only work for low speeds less than 10mph when rolling resistance and wind resistance are small. Also bear in mind that you need to consider a power level that is sustainable for over an hour.

I've worked out climbing speeds up each of the climbs in turn. I've done this for three weights of rider + bike - 200lb (= 90.5Kg), 225lb (= 101.8 Kg ) and 175lb (= 79.2Kg). Click here to view the spreadsheet.