That sausage McGriddle I ate this morning....ugh. Last time I let my wife bring breakfast to me at the office! Felt like I was gonna sink at my afternoon swim! Now I remember why I hat McDonalds.

bonking 7 miles from home and hitchhiking back. that was a good one.

Biggest mistake I made was too little training at the beginning of the season and too much near the end (This is a big one if you want to race standard distance - Pre-season should be big miles, low intensity. Months leading up to a goal event should be low miles high intensity).

Also another mistake which is pretty common is going way too hard too early in the season and not hard enough near your goal event. (Which in turn will make you a "chugger" - someone who can only race at one pace)

Mark.

Trying to make up missed workouts.
Go too hard on 'easy' days
Not hard enough on the 'hard' days
Not getting enough rest - this is still a problem.

Running because it was pooring and I couldn't do the bike I was supposed to do . . . after the run had beer and guacamole with my roomies, then got an invite to bike when the rain unexpectedly cleared. My stomach really didn't enjoy that bike ride, but now I know that I can survive it. ( I guess that's a pretty obivious mistake)

Really not resting enough at the right times is the biggest one, but after 6 years of serious distance training feeling that has taught me where the line is and what happens when I try to tightrope it.

Overtraining and getting hurt. Last year at the beginning of training I had a brilliant idea that I can run far because I feel so good. Big mistake leading to patella tendinitis which still bothers me about a year later.

Two years ago I put a 12 mile run after a 30+ mile bike. My longest run up to that point was about 9 miles and that was a run only day. This is when my first bout of ITBS started. By the time I walked home it was noon, 90+ degrees, I was out of fuel, hydration, I had the chills, everything. Completely BONKED! Worst feeling I've ever experienced. That 12 mile run took me almost 3 hours. UGH!

Training with a heart rate monitor is probably the best thing you can do to get more efficient with your training. That was my major mistake when I first started training two years ago, I would just go out and train without any structure. I wouldn't train in any zones and just played each day by how I felt. There was no plan. This is what hurts beginning triathletes.

Always start with a structured plan and stick to it as best as you can!

Thanks so far for all the the replies. Keep them coming!