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Just go with this: Jennifer Aniston and her beau of four months Justin Theroux are on the fast track to walk down the aisle...and procreate!

As Aniston, 42, and Theroux (who turns 40 on Wednesday) kick back on an extended getaway in a $7,000-a-day cliffside compound in Kauai, Hawaii, the hot twosome have more on their minds then coconuts, cocktails and outdoor yoga.

"They have talked at length about getting married and starting a family," an insider close to the pair tells the new Us Weekly, out Wednesday. "She is anxious for the next phase of her life and feels like this is the time."

And her hunky new guy (who split with girlfriend of 14 years Heidi Bivens in March) is completely on board with having a baby, a second insider says: "They both want it to happen soon."

To that end, health nut Aniston is making sure her body is in tip-top shape to conceive while on the island -- hiring a private chef and doing at least 45 minutes of yoga daily. "She's detoxifying," an island source says.

The Horrible Bosses star's pals aren't surprised in the slightest by Aniston's race to become a mom with Theroux. "Jen hasn't felt this way about anyone since Brad," a pal says of Aniston's ex-hubby Brad Pitt. (Aniston has had a famously rocky romantic life -- with fizzled romances with Vince Vaughn, John Mayer and a slew of others -- since her 2005 split with Pitt, 47.)

Adds another friend: "They are moving quickly, but they know the feelings they have about [each other] are different from past romances."

For much more on Aniston and Theroux's quest to have a baby -- how she's found her "soulmate," how Theroux makes "all of her insecurities melt away," and their house-hunting plan in L.A. -- pick up the new Us Weekly, out now!