Trying hard enough is relative. There are different approaches and all are valid if they work for the person using. Some like to overshoot figuring if they don't make the goal at least they got further than they would have setting a low expectation. The potential problem with this approach is to never meet one's goals which some may find demoralizing. Some like to set lower goals with a better chance to exceed the mark, which has the potential problem of underachieving within one's given potential.
The trick if trying to meet some performance related goal is to make those miles really count. I would say 100 miles of quality with some time/intensity markers is better than 150 just riding around. If looking to improve consistency matters above all else. Someone doing an 1 or 1.5 hour rides four or five days a week will probably improve more than someone doing 2 hour rides here and there. Even someone doing hour long rides in specific training zones consistently can easily see improvement.
In the colder months I try for at least an hour on the rollers a few times a week. Once riding weather rolls into town I try to get in four or five days a week with one longer ride (about 50 miles), one brisk ride alternating hot/cold laps in my local park which has a 3.5 mile loop and each takes me about 9 minutes in my sweet spot (a regular workout is two laps on and one off), and then maybe one day after having warmed up at endurance pace for an hour or so when I'm really on the gas for 3, 4, or 5 minutes at a time for several efforts separated with easier pedaling. That 's one day of endurance, one day of sub-threshold, and one day of VO2 . Then whatever else happens on the other day or two, happens... maybe a race, maybe a fun ride with the group going around the park, or just a super easy ride after a night at the bar. It actually works out pretty well and falls into "undulating periodization". I don't reach my max potential this way but all my energy systems are for the most part ready for a spontaneous race if one pops up (these days if I race it's purely for fun). If I'm really busy I might skip the longer ride but will still try to put in an hour, and if I haven't been riding as much as I'd like I skip the intensity. Overall it's a pretty easy strategy to meet.
I had the goal this year of racing at Battenkill and was on a structured plan and doing pretty well, unfortunately some derailing due to job hunting and a stint in the hospital interrupted things. I wasn't able to make it (the race is actually taking place today), and although a little disappointed I still ended up with some nice fitness, more than I usually have by this time of year, so the cloud has somewhat of a silver lining.
