Jennifer Aniston, Justin Theroux Trying for a Baby!

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!

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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."

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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.

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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."