[El Gato]

And since Eisenhower, the most militarily experienced men elected President were Jimmy Carter and George H.W. Bush, neither of whom are spoken among the all-time greats.

Nixon and Ford had as much real experience as Carter. All three were Navy officers. Ford was on a carrier, but not a flyer. Nixon served on active duty for almost 4 years from '42 to '46. Carter was a submariner but didn't graduate from the Naval Academy until after WW-II was over.

Nixon:

5 JUN 1942 Appointed Lieutenant (junior grade) in U.S. Naval Reserve 1 OCT 1943 Lieutenant
3 OCT 1945 Lieutenant Commander
10 MAR 1946 Relieved of active duty
1 JUN 1953 Commander in Naval Reserve
1 JUN 1966 Retired from Naval Reserve

Ford:

13 April 1942 Commissioned Ensign, US Naval Reserves
20 April 1942 - May 1942 Under instruction, US Naval Academy
May 1942 - May 1943 US Navy Pre-Flight School, Chapel Hill, North Carolina
May 1943 - June 1943 Outfitting and commissioning of USS Monterey (CVL-26) , New York Shipbuilding Corporation, Camden, New Jersey
June 1943 - December 1944 Assistant Navigator on USS Monterey (CVL-26)
December 1944 - April 1945 Athletic Department of Navy Pre-Flight School, St. Mary's College, California
April 1945 - January 1946 Staff Physical and Military Training Officer, Naval Reserve Training Command, Naval Air Station, Glenview, Illinois
January 1946 - February 1946 Separated at Great Lakes Naval Training Center, Great Lakes, Illinois
(Carter also separated as a Lt. Commander, but that was actually a quicker rise than Nixon, who started as a J/G rather than an Ensign, and separated about the same time.)

Carter

# 05 JUN 1946 - Commissioned Ensign, USN.
# 8 AUG 1946 - 23 JUL 1947 -- Duty aboard USS Wyoming (E-AG17).
# 23 JUL 1947 - 14 JUN 1948 -- Duty aboard USS Mississippi (E-AG128).
# 14 JUN 1948 - 17 DEC 1948 -- Duty under instruction at the Officer's course, USN Submarine School, Submarine Base, New London
# 17? DEC 1948 - 01 FEB 1951 -- Duty aboard USS Pomfret (SS-391)
# 05 JUNE 1949 -- Promoted to Lieutenant (j.g.)
# 01 FEB 1951 - 10 NOV 1951 -- Duty with Shipbuilding and Naval Inspector of Ordnance, Groton, CT as prospective Engineering Officer of the USS K-1 during precommissioning fitting out of the submarine.
# 10 NOV 1951 - 16 OCT 1952 -- Duty aboard USS K-1(SSK-1) Submarine was new construction, first vessel of its class # 01 JUNE 1952 -- Promoted to Lieutenant
# 16 OCT 1952 - 08 OCT 1953 -- Duty with US Atomic Energy Commission
From 1 MAR 1953 to 8 OCT 1953 he was under instruction to become an engineering officer for a nuclear power plant. He also assisted in setting up on-the-job training for the enlisted men being instructed in nuclear propulsion for the USS Seawolf (SSN575).
9 OCT 1953 -- Honorably discharged

So Carter did have more active duty time, but Nixon and Ford served during wartime.

[El Gato]

Oh, and GHW Bush served on active duty for less time than any of them. But he was also the youngest by far, and the only one to enlist. he served for 10 months as an enlisted sailor. He was the youngest navy pilot at the time.

GHW Bush:

13 JUN 1942 Enlisted in U.S. Naval Reserve
5 AUG 1942 Reported for Active Duty
8 JUN 1943 Honorably Discharged
9 JUN 1943 Ensign, U.S. Naval Reserve and continued on Active Duty
1 AUG 1944 Lieutenant (junior grade)
18 SEP 1945 Released from Active Duty under honorable conditions
16 NOV 1948 Lieutenant

So while Carter had the most military active duty, GHW Bush had the least.