Is 80% good enough?

Katie,

I assume you're talking about following a canned training plan like one here on BT rather than a custom plan created by a coach.  Canned plans don't take into account your specific circumstances, including whether Sunday of the 17th week is Mother's day, so you won't be doing your 3+ hour run that day in spite of what the plan says.  

Train consistently and let lost workouts go whether it's because of health, weather, or other commitments.  Adjust to keep your long and key workouts as much as you can, but even if you miss a long run or a long ride, you'll still do fine at the race.  

As you get closer to the race, formulate your race strategy based on what you have done in training.  If you only hit 50% of your long runs, back off the bike a little and take your time in the first half of the run.  Maybe adopt a run/walk strategy.  What doesn't work is to go into the race having hit 50% of your runs and race as if you fully trained.  

Details please. For example, what was last weekend's planned workout and what did you actually do? What is this weekend? Are you skipping whole workouts or shorting some?

The point of most long workouts is the last part, where the overload happens that results in adaptation. As long as you reaching overload you are generally getting better. You need enough volume, but most plans have more then you need so skipping some is OK. Consistently shorting won't give the results you want.

there's nothing in your training log, so, kinda' hard to tell.  The impact of this will be dependent on your total volume and your goals for race day.  Not what % of a particular plan you've met.  80% of 15hrs/wk is greater than 100% of 10hrs/wk.