PeakO2 is a great ingredient. Can't go wrong there.
I'd also look at Olympus Labs
Endur3+...it's almost tailor-made for this.
You get a huge dose of
EAA/BCAA which will help stave off catabolism during long bouts of exertion, increase immediately available energy stores, and perhaps, most importantly, enhance post-exercise protein synthesis:
Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances postexercise muscle protein synthesis.
https://www.ncbi.nlm.nih.gov/pubmed/21775557
You get
Taurine to help reduce muscle cramping and soreness,
Glutamine to prevent both peripheral and central fatigue by its action on blood ammonia concentration.
Glutamine and carbohydrate supplements reduce ammonemia increase during endurance field exercise
https://www.ncbi.nlm.nih.gov/pubmed/18059593
Next up is a biggie:
20 grams of Carb10, a fast-digesting, yet low glycemic carb source that will keep you fueled during long runs. Carbs are arguably the single greatest ergogenic aid an athlete has, and Carb10 will give you what you need without weighing you down or making you feel bloated.
Moving on we've got
Hydromax and
Agmatine that can both help reduce fatigue and also improve endurance. Hyperhydration with Hydromax combined with water was found to be far superior to water alone and significantly increased fluid retention by approximately 900mL over placebo. You also get
Schisandra for its ability to reduce stress, increase blood flow and stimulate glucose uptake, and a real standout:
(-)/-Epicatechin which is a favorite for boosting endurance as it can enhance exercise and oxidative capacity and increase angiogenesis (creation of blood vessel formation).
Finally, you get a well-dosed
Hydration matrix that provides you with the full set of electrolytes; Sodium Chloride, Potassium Gluconate, Magnesium Aspartate and Calcium Citrate. Also included is 500mg of coconut water to maintain hydration, prevent cramping and reduce fatigue.
As you can see, Endur3+ will assist your marathon-training from a variety of angles and pathways, allowing you to run farther, faster, and recover better. It's currently on sale for
25% off at Nutriverse with code "AM5". You'll get all the aforementioned goodness for only $26.99. Grab a bottle and say hello to improved times.