Kim it is SO worth it, i'm telling you. I just did a 6k with DD in the buggy and that tells me i'm fit for at least 8k without her. Seriously it makes that much difference. It's like running with weights. Burns way more calories, tones the thighs and calves and bum even more, great for your heart (which is why it makes you feel like you want to die, lol) and is all round great for you. Keep on keeping on. What i would do if i were you is do a week of workouts with the buggy and then do one without, you will SEE and FEEL how far you've come.
*giant pats on backs of all you hard-runnin' ladies*
Bx