Wiffen became the first Irishman to win a gold medal in swimming with his victory in the 800m freestyle on Tuesday and his time of 14:40.34 was 2.22 quicker than Italy's Gregorio Paltrinieri.
"I'm not gonna lie. I felt like I was carrying a bit of fatigue. I haven't really slept properly since the 800," said Wiffen.
"(At) The start I was a bit uncomfortable and then I really set on to the pace, and then I was actually kind of surprised when it felt to me like a 1450 swim and it was a 1440," he said.
Wiffen -- who also plans to swim in the 10k marathon open water race next week -- said he has tried to keep his gold medal success out of his mind as he prepares for the 1500m.
"I'm so happy but to be honest I've had to park it in the back of my mind going into this race. I'm looking at it as if I haven't won anything and I'm going in the same attitude I went in with the 800 final," he said.
The 23-year-old said he believed he would need to break the world record time of 14:31.02, set by Sun Yang in the London Games in 2012, if he was to complete the double.
"I'll give you a little insight, I'm going to try have a crack at this world record. If it happens, it happens, if it doesn't, it doesn't but I think personally it's gonna take that to win gold," he said.
'Different ways to go fast'
Wiffen said that his original target for Paris was to win in the longer of the two distance events.
"I really want this 1500 gold. I came into the meet and that was my aspiration, the 800 I wasn't really thinking about too much because I love the 1500," he said.
"But coming into the 1500 now I have to find different ways to motivate myself and different ways to go fast. That's exactly what I'm going to do tomorrow (Sunday) night," he said.
In the women's 50m freestyle Sweden's Sarah Sjoestroem, the Paris gold medallist in 100m freestyle, who won silver in 50m free in Tokyo, was fastest in Saturday's heats.
Sjoestroem moved into the semi-finals with a time of 23.85 with Poland's Katarzyna Wasick and American Gretchen Walsh the next quickest.
"It was really good. I don't think I've ever been that fast in a prelim at this kind of competition, like Olympics or World Champs, so yeah, I'll take that," said the Swede.
French star Leon Marchand -- fresh from winning his fourth gold of the Games in Friday's 200m individual medley -- was back in the pool and led the French team to the fastest time in qualifying for the men's 4x100m medley relay.
China were second fastest, 0.22 behind France, with defending champions the USA third quickest.
In the women's race, defending champions Australia were quickest with a time of 3:54.81, finishing 1.29 ahead of Canada.
