I'm pretty inexperienced but will relate you two anecdotes of my own:
In 2008 I tried to ride an 8 day end-to-end (i.e. something over 100 miles a day) and failed. Didn't have the fitness (or experience) and succumbed to injury at halfway only!
About six months ago I comfortably rode my first 600k brevet (the Border Raid) on a pretty slow vehicle - a recumbent trike. This was not dissimilar from the distance I managed on my 4 days of e2e attempt in one go.
Before the end-to-end attempt I had a regular diet of 175 mile weeks (35 miles a day of mainly open road commuting ~ 2 hours a day).
Before the 600k I was riding just 8 miles a day to work, slowly through the city centre. But I did have six 200km brevets under my belt, which had got me used to riding the longer distances.
So, FWIW I'm not convinced you should prioritise intensity too much, unless you are definitely getting in longer rides anyway. Personally, I'm (trying to) get in three quality sessions of aerobic work a week, with a view to moving on to longer rides when we get into audax season, at which point I'm going to switch my midweek routine to harder pace.