What route are you taking from Valdez to Denali? I assume up the Richardson and then over the Denali Hwy at Paxson? Or will you cut across the Glenn Hwy from Glennallen to the Parks Hwy then to Denali? If you aren't including the Glenn Hwy, you might want to consider it. It's a very pretty road, especially riding west to east (Anchorage to Glennallen). If you aren't stuck on seeing Denali, a great route would be Anchorage to Glennallen along the Glenn Hwy, then take the Richardson south to Valdez.
60 miles is certainly doable for experienced riders, but don't underestimate the climbing on the route. There aren't any huge climbs, but lots of rolling hills, especially out of Valdez. If you are doing the ride self-contained (even w/o camping gear), that might be optimistic on a tandem (I say this as a person who has done bike tours on a tandem - and triplet - since 1999). I'm pretty familiar with that area as I lived in Glennallen, Alaska, for several summers and regular drove and rode all the roads around there. If you've never been to Alaska, budget in lots of time to stop and take photos! Of course, the good think is you have nearly unlimited daylight in case you need more time to ride.
If you are riding the Denali Hwy from Paxson to Cantwell, then 60 miles is VERY optimistic, as it is all gravel and rolling terrain.