Predicting race performance

Here's an interesting question for you more experienced athletes.

I'm a 46 year old male, recreational (4:30-4:45) marathoner, who's trying a Sprint Tri in a few weeks. From my experience in running, I think it's important to have a time goal, rather than bolting out at a pace you can't sustain. While I've got a good handle on my performance capabilities for the individual elements of the tri, I'm not sure how that'll translate into race day performance when I've got to combine them all. Here's my breakdown:

Swimming (750M in the event) -- I generally can do 7 to 8 100 yd repeats with 30 sec rest at 1:45 pace. I swim twice a week at Master's classes, usually about 2000 yds per session, so 4000 yds a week.

Cycling (17 miles in the event) -- Terrain here in VA is hilly. I've recently done a 17 mile ride at 17 mph with significant climbing, and do longer weekend rides (30 miles) at about 15 mph. I've been training at about 40-50 miles a week

Running (5K in the event) -- Recently did a 23:58 5K, and can pretty easily click off 6 miles at 9 min/mile. Currently running about 10-12 miles a week.

I'm thinking I can do the whole thing (750M/17 mi/5K) in about 1:40, given transition time and a more moderate pace on the individual elements. I'm hoping to finish around the middle of my age group, based on local results.

Any opinions?