What are your goals for this 70 mile ride? Are they to survive it or do it in a certain no of hours? Does it include mtn passes or is it on the trails around Denver?
Last week I did a 370 mile, 7 day tour of N. Dakota. Mileage most days was 60-70 miles. My goal was to survive the winds, cross and head, so I focused on endurance. Because it was a tour I picked up the last 4 weeks of the training plan for Ride the Rockies. I logged 600 miles in July but the longest ride I did was 54 miles. The week before the ride I tapered my mileage. I survived the wind and the week of riding without issue. I wasn't the fastest rider but I was well prepared for long days on the bike and didn't struggle with the hills or the wind.
A training plan that I have used for a century suggests 4-6 days of riding, most in the 45-90 min per day range with tempo rides, 1 long ride (3-4 hrs) and an easy recovery day. Joe Friel says there are 3 components of training, Frequency (how often to ride) Intensity (how hard to ride) and Time (how long to ride). You will need to work out what is right for you.
Tailor your training rides to the terrain and conditions you will be riding. I did Ride the Rockies last year and did some climbing on passes, Vail Pass and Montezuma outside of Keystone but I have not ridden Lookout Mtn or Mt. Evans. The day we rode over Rabbit Ears pass I climbed it with flat tire. I had friends who rode Mt. Evans as part of their training for the N. Dakota trip but died in the wind and had to sag in. BTW, they had no specific training plan.
Also, ride consistently. I notice many riders stop pedaling and coast, often before they reach an uphill. Every time you coast you lose speed and have to catch up. On a long ride don't burn your candles to fast. Many riders take off fast and stay anaerobic too long. Then they have nothing left at the end of the ride.
On your long rides experiment with food, what agrees with you, how much you need etc. Energy drinks are great but don't agree with everyone. I use Accelerade, can't survive more than 2 hrs without it but I also carry Cliff bars and a peanut butter sandwich with me, just in case. Bonking on a long ride is not fun. Many roadies think that Camelbaks are not cool but I use one because I drink more frequently and the Accelerade stays fresher in the pack than in a water bottle.
If you've already done 50 miles and it wasn't a struggle then 70 won't be a big deal.