How to speed up this "closest records" mySQL query

Question

I'm trying to see if I can speed up this query with an index (or other tweaks). Basically, I want to find the 6 closest records based on Latitude and Longitude in the glinks_Links table.

SELECT
    ID, Ville, Title, Telephone, ExpiryDate,
    Address, Postal_Code, Latitude, Longitude, priceSort, priceSortBNB, priceSortGITE, has_image,

    ( 6371 * acos( cos( radians('42.934817491341060000000000000000') ) * cos( radians( Latitude ) ) * cos( radians( Longitude ) - radians('2.222607135772705000000000000000') ) + sin( radians('42.934817491341060000000000000000') ) * sin( radians( Latitude ) ) ) ) AS distance
    FROM glinks_Links link
        WHERE
            link.ExpiryDate < 2147483647
            AND isValidated = "Yes"
            AND OffersLinkID IS NULL
            AND (
            ((Latitude BETWEEN (42.934817491341060000000000000000 - 35/69.0) AND (42.934817491341060000000000000000 + 35/69.0)) )
            AND
            (Longitude BETWEEN (2.222607135772705000000000000000 - 35/42.5) AND (2.222607135772705000000000000000 + 35/42.5))
            )
        HAVING
            distance < 25
        ORDER BY distance
        LIMIT 6

The table structure (remove all the extra stuff thats not in the query, for simplicity), is:

CREATE TABLE `glinks_Links` (
  `ID` int(10) UNSIGNED NOT NULL,
  `Title` varchar(100) NOT NULL DEFAULT '',
  `isValidated` enum('No','Yes') NOT NULL DEFAULT 'Yes',
  `ExpiryDate` int(11) NOT NULL DEFAULT '2147483647',
  `Address` varchar(100) DEFAULT NULL,
  `Postal_Code` varchar(30) DEFAULT NULL,
  `Ville` varchar(50) DEFAULT NULL,
  `Latitude` decimal(34,30) DEFAULT NULL,
  `Longitude` decimal(34,30) DEFAULT NULL,
  `OffersLinkID` int(11) DEFAULT NULL,
  `priceSortBNB` int(11) DEFAULT NULL,
  `priceSortGITE` int(11) DEFAULT NULL,
  `priceSort` int(11) DEFAULT NULL,
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

It only takes 0.03 seconds to run, but I have to do it 50k or so time, so it all adds up. Any suggestions are much appreciated :)

UPDATE: I should probably mention I already have some indexes on the table:

ALTER TABLE `glinks_Links`
  ADD PRIMARY KEY (`ID`),
  ADD KEY `expiryndx` (`ExpiryDate`),
  ADD KEY `valexpndx` (`isValidated`,`ExpiryDate`),
  ADD KEY `Latitude` (`Latitude`,`Longitude`);