Startup Training - Swim=OK Run/Bike=Fatigue

Greetings everybody!

Time to ask for advice...

Situation: Month one of the Couch to Olympic - Summer is starting where I live (Argentina) so I'll miss this Tri season - it gives me some more months to train. I'm above the minutes for swimming but seriously below for run and bike.

Age/height: 39/5'4" - With aprox. 8 lb of excess body baggage.

Swim: OK - I've maintained some level of activity since my college years. These days I swim 2000 to 2400mts 3 times a week and I can consistently complete 1500 in 28-33min (depending on lane traffic)

Run: Mixed treadmill/outdoor - Currently, I can maintain a 16 min jog until I feel very fatigued (I've been increasing this from 10 min during the last 4 weeks)

Bike: Mostly indoor bike and yesterday I took a spinning class.

Question/request for advice: Yesterday I ran 16 min, walked 14 min and took the 45 min spinning class. I was very fatigued while running and spinning - sweated profusely. Today I don't feel lactic acid buildup or pain from having overworked my legs - very little soreness.

Is it that I just need to continue developing my leg muscles? Aerobic ability? Something else? Any suggested activity to improve my endurance and ability to run?

Thanks.