I'm with Koffee & Layton on this one, you gotta eat more. That 2700-3100 calorie diet is fine for someone of your height and weight if you are a completely sedentary office worker. You'll burn a lot more calories than most people just in keeping your body warm, in the arena of 2000-2300 calories (I'm assuming you're between 4-10% body-fat). Doesn't leave much for metabolism and exercise...
How long have you been riding? I'm assuming for more than 3-months and your pace on a hilly 18.5 mile shows you're in good shape. This means that you can burn off a large number of calories/hour, probably in the range of 700-900/hr on that ride. And you're doing weight-workouts, which will tax your muscles at a higher peak load than any kind of biking.
What's causing your weakness is this:
1. lack of carbs on rides, spare protein is used for fuel, then muscles are disassembled
2. lack of carbs after rides causes body to disassemble muscle to restore glycogen supply
3. weight workouts on non-cycling muscles causes disassembly of other muscles (legs) to repair upper-body and back
Here's what you can do in the next couple weeks and I guarantee you'll feel MUCH stronger and can ride MUCH faster within just 1 month:
1. eat more - you'll need about 3600-3900 calories per day minimum, depending upon your workout schedule. This means 1/2 extra serving of rice immediately after a ride within 15-minutes. And another extra serving in a meal within 2-3 hours. Also eat at least 1-2 energy-bars/gels (150-200cal/each) on your rides.
2. add more intensity to leg workouts - cut back the rides to 4 days per week. Make one day super-short of 15-miles, no more, and do 4-5 all-out 100% maximum-effort, maximum-RPM screaming sprints. If you're not screaming, you're not pushing hard enough
. Go as hard as you can for as long as you can, about 35-45 seconds. Get full recovery between sprints. Then go home...
3. work on leg-strength in gym - Change one of your gym days to work on legs. Do two weeks of medium-intensity 75-80% of max-lift @ 5-10 reps on the legs. On the 2nd two weeks, do high-intensity 80-95% of max-lift @ 2-7 reps on legs. You can do this before your endurance ride. Try to get one full rest day after your intense leg workout.
4. do one endurance ride per week on bike. This will make your energy-system more efficient at delivering to your muscles and burn more fat (reduces the muscle catabolism). Try to do a 45-55 mile ride once a week. Eat a meal before you leave, and eat at least 3 energy-bars/gels one the ride, 1 per hour. Drink lots.
5. take a rest week - on the 5th week, cut back to 3 days of riding and 2 days of light lifting. Still do 1 day of sprints (up to 5-6 sprints now) and 1 day of endurance 50-60 miles. Try to get 1 full rest day every other day.
After this 5th week, you'll feel like SUPERMAN! Do that hilly 18.5 mile ride again and you'll have taken 5-minutes off. You'll be able to sprint with SNAP, you'll feel like you can rip the handlebars and cranks in half!
That's it in a nutshell, EAT more, RIDE HARDER, and LONGER (just not on the same day). You were caught in no-mans-land of training; stuck in the middle.